3月份leetcode

Author： kok-s0s
发布时间：March 31, 2021
2796 views
24939 words
Categories：码无止境

303. 区域和检索 - 数组不可变

前缀和解决此题，做个差得出一个序列里的总和值。

class NumArray {
public:
    vector<int> preSum;

    NumArray(vector<int>& nums) {
        int n=nums.size();
        preSum.resize(n+1);
        // int *preSum=new int[n+1];
        for(int i=0;i<n;++i)
            preSum[i+1]=preSum[i]+nums[i];
    }

    int sumRange(int i, int j) {
        return preSum[j+1]-preSum[i];
    }
};

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray* obj = new NumArray(nums);
 * int param_1 = obj->sumRange(i,j);
 */

304. 二维区域和检索 - 矩阵不可变

learn

class NumMatrix {
public:
    vector<vector<int>> preSum;

    NumMatrix(vector<vector<int>>& matrix) {
        int rows=matrix.size();
        if(rows>0) {
            int cols=matrix[0].size();
            preSum=vector<vector<int>> (rows+1,vector<int>(cols+1,0));
            for(int r=0;r<rows;++r) {
                for(int c=0;c<cols;++c) {
                    preSum[r+1][c+1]=preSum[r+1][c]+preSum[r][c+1]-preSum[r][c]+matrix[r][c];
                }
            }
        }
    }

    int sumRegion(int row1, int col1, int row2, int col2) {
        return preSum[row2+1][col2+1]-preSum[row2+1][col1]-preSum[row1][col2+1]+preSum[row1][col1];
    }
};

/**
 * Your NumMatrix object will be instantiated and called as such:
 * NumMatrix* obj = new NumMatrix(matrix);
 * int param_1 = obj->sumRegion(row1,col1,row2,col2);
 */

338. 比特位计数

learn

// 线性时间O(n)内用一趟扫描 
// 算法的空间复杂度为O(n)
class Solution {
public:
    vector<int> countBits(int num) {
        vector<int> bits(num+1);
        for(int i=1;i<=num;++i) {
            bits[i]=bits[i>>1]+(i&1);
        }
        return bits;
    }
};


// 时间复杂度为O(n*sizeof(integer))
// class Solution {
// public:
//     int getNumber(int x) {
//         int count=0;
//         do {
//             if(x%2==1)
//                 count++;
//             x/=2;
//         }while(x);
//         return count;
//     }
//     vector<int> countBits(int num) {
//         vector<int> ans;
//         for(int i=0;i<=num;++i) {
//             ans.push_back(getNumber(i));
//         }
//         return ans;
//     }
// };

354. 俄罗斯套娃信封问题

learn

没做出来

class Solution {
public:
    int maxEnvelopes(vector<vector<int>>& envelopes) {
        if (envelopes.empty()) {
            return 0;
        }

        int n = envelopes.size();
        sort(envelopes.begin(), envelopes.end(), [](const auto& e1, const auto& e2) {
            return e1[0] < e2[0] || (e1[0] == e2[0] && e1[1] > e2[1]);
        });

        vector<int> f(n, 1);
        for (int i = 1; i < n; ++i) {
            for (int j = 0; j < i; ++j) {
                if (envelopes[j][3] < envelopes[i][4]) {
                    f[i] = max(f[i], f[j] + 1);
                }
            }
        }
        return *max_element(f.begin(), f.end());
    }
};

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/russian-doll-envelopes/solution/e-luo-si-tao-wa-xin-feng-wen-ti-by-leetc-wj68/
来源：力扣（LeetCode）
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

232. 用栈实现队列

My Solution

class MyQueue {
public:
    stack<int> a,b;
    /** Initialize your data structure here. */
    MyQueue() {

    }

    /** Push element x to the back of queue. */
    void push(int x) {
        a.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int temp;
        while(!a.empty())
        {
            b.push(a.top());
            temp=a.top();
            a.pop();
        }
        b.pop();
        while(!b.empty())
        {
            a.push(b.top());
            b.pop();
        }
        return temp;
    }

    /** Get the front element. */
    int peek() {
        int temp;
        while(!a.empty())
        {
            b.push(a.top());
            temp=a.top();
            a.pop();
        }
        while(!b.empty())
        {
            a.push(b.top());
            b.pop();
        }
        return temp;        
    }

    /** Returns whether the queue is empty. */
    bool empty() {
        if(a.empty()&&b.empty())
            return true;
        else
            return false;
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */

Official solution

class MyQueue {
private:
    stack<int> inStack, outStack;

    void in2out() {
        while (!inStack.empty()) {
            outStack.push(inStack.top());
            inStack.pop();
        }
    }

public:
    MyQueue() {}

    void push(int x) {
        inStack.push(x);
    }

    int pop() {
        if (outStack.empty()) {
            in2out();
        }
        int x = outStack.top();
        outStack.pop();
        return x;
    }

    int peek() {
        if (outStack.empty()) {
            in2out();
        }
        return outStack.top();
    }

    bool empty() {
        return inStack.empty() && outStack.empty();
    }
};

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/implement-queue-using-stacks/solution/yong-zhan-shi-xian-dui-lie-by-leetcode-s-xnb6/
来源：力扣（LeetCode）
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

503. 下一个更大元素 II

class Solution {
public:
    vector<int> nextGreaterElements(vector<int>& nums) {
        int n=nums.size();
        vector<int> ans(n,-1);
        stack<int> Sta;
        for(int i=0;i<n*2-1;++i) {
            while(!Sta.empty()&&nums[Sta.top()]<nums[i%n]) {
                ans[Sta.top()]=nums[i%n];
                Sta.pop();
            }
            Sta.push(i%n);
        }
        return ans;
    }
};

class Solution:
    def nextGreaterElements(self, nums: List[int]) -> List[int]:
        n = len(nums)
        ret = [-1] * n
        stk = list()

        for i in range(n * 2 - 1):
            while stk and nums[stk[-1]] < nums[i % n]:
                ret[stk.pop()] = nums[i % n]
            stk.append(i % n)

        return ret

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/next-greater-element-ii/solution/xia-yi-ge-geng-da-yuan-su-ii-by-leetcode-bwam/
来源：力扣（LeetCode）
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

131. 分割回文串

回溯算法

class Solution {
    List<List<String>> res = new ArrayList<>();

    public List<List<String>> partition(String s) {
        backTrack(new ArrayList<>(), 0 ,s);
        return res;
    }

    private void backTrack(List<String> preChoice, int curStep, String s) {
        if(curStep == s.length()) {
            res.add(new ArrayList<>(preChoice));
            return;
        }

        for(int i = curStep; i < s.length(); i++) {
            String str = s.substring(curStep, i + 1);
            if(isHuiWen(s, curStep, i)) {
                preChoice.add(str);
                backTrack(preChoice, i+1, s);
                preChoice.remove(preChoice.size() - 1);
            }
        }
    }

    private boolean isHuiWen(String s, int left, int right) {
        while(left < right) {
            if(s.charAt(left) != s.charAt(right)) {
                return false;
            }
            left++;
            right--;
        }
        return true;
    } 
}

DP操作

class Solution {
    List<List<String>> res = new ArrayList<>();

    public List<List<String>> partition(String s) {
        int N = s.length();
        if(N == 0)  return res;
        List<String> init = new ArrayList<>();
        init.add(s.substring(0,1));
        res.add(init);
        for(int i = 1; i < N; i++) {
            String cur = s.substring(i, i + 1);
            List<List<String>> tempRes = new ArrayList<>();
            for(List<String> item : res) {
                int n = item.size();
                /*第一种情况，直接加入当前字符*/
                List<String> newItem1 = new ArrayList<>(item);
                newItem1.add(cur);
                tempRes.add(newItem1);

                /*第二种情况，当字符与最后一个字符串拼接可以构成新的回文串*/
                List<String> newItem2 = new ArrayList<>(item);
                if(item.get(n-1).equals(cur)) {
                    newItem2.set(n - 1, cur + cur);
                    tempRes.add(newItem2);
                }
                /*第三种情况，当前字符利用最后一个字符串，与倒数第二个字符串结合，构成回文串*/
                List<String> newItem3 = new ArrayList<>(item);
                if(n >= 2 && item.get(n-2).equals(cur)) {
                    String last = item.get(n - 1);
                    newItem3.remove(n-1);
                    newItem3.set(n - 2, cur + last + cur);
                    tempRes.add(newItem3);
                }
            }
            res = tempRes;
        }
        return res;
    }
}

Official solution

class Solution {
    boolean[][] f;
    List<List<String>> ret = new ArrayList<List<String>>();
    List<String> ans = new ArrayList<String>();
    int n;

    public List<List<String>> partition(String s) {
        n = s.length();
        f = new boolean[n][n];
        for (int i = 0; i < n; ++i) {
            Arrays.fill(f[i], true);
        }

        for (int i = n - 1; i >= 0; --i) {
            for (int j = i + 1; j < n; ++j) {
                f[i][j] = (s.charAt(i) == s.charAt(j)) && f[i + 1][j - 1];
            }
        }

        dfs(s, 0);
        return ret;
    }

    public void dfs(String s, int i) {
        if (i == n) {
            ret.add(new ArrayList<String>(ans));
            return;
        }
        for (int j = i; j < n; ++j) {
            if (f[i][j]) {
                ans.add(s.substring(i, j + 1));
                dfs(s, j + 1);
                ans.remove(ans.size() - 1);
            }
        }
    }
}

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/palindrome-partitioning/solution/fen-ge-hui-wen-chuan-by-leetcode-solutio-6jkv/
来源：力扣（LeetCode）
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

132. 分割回文串 II

打※题 important

class Solution {
    public int minCut(String s) {
        int n = s.length();
        boolean[][] g = new boolean[n][n];
        for (int i = 0; i < n; ++i) {
            Arrays.fill(g[i], true);
        }

        for (int i = n - 1; i >= 0; --i) {
            for (int j = i + 1; j < n; ++j) {
                g[i][j] = s.charAt(i) == s.charAt(j) && g[i + 1][j - 1];
            }
        }

        int[] f = new int[n];
        Arrays.fill(f, Integer.MAX_VALUE);
        for (int i = 0; i < n; ++i) {
            if (g[0][i]) {
                f[i] = 0;
            } else {
                for (int j = 0; j < i; ++j) {
                    if (g[j + 1][i]) {
                        f[i] = Math.min(f[i], f[j] + 1);
                    }
                }
            }
        }

        return f[n - 1];
    }
}

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/palindrome-partitioning-ii/solution/fen-ge-hui-wen-chuan-ii-by-leetcode-solu-norx/
来源：力扣（LeetCode）
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

1047. 删除字符串中的所有相邻重复项

//2020-4
class Solution {
public:
    string removeDuplicates(string s) {
        char stack[20002];
        int top=-1;
        for(int i=0;s[i]!=0;++i)
        {
            if(top!=-1)
            {
                if(s[i]==stack[top])
                {
                    --top;
                }
                else{
                    stack[++top]=s[i];
                }
            }
            else
            {   
                stack[++top]=s[i];
            }
        }
        string a;
        for(int i=0;i<=top;i++)
            a+=stack[i];
        return a;
    }
};

class Solution {
public:
    string removeDuplicates(string S) {
        string sta;
        for(char ch:S) {
            if(!sta.empty()&&sta.back()==ch)
                sta.pop_back();
            else    
                sta.push_back(ch);
        }
        return sta;
    }
};

224. 基本计算器

使用一个栈解决，对括号和加减号进行处理。

class Solution {
public:
    int calculate(string s) {
        stack<int> Sta;
        Sta.push(1);
        int flag=1;

        int ans=0;
        int n=s.length();
        for(int i=0;i<n;) {
            switch(s[i]){
                case ' ':{
                    ++i;
                    break;
                }
                case '+':{
                    flag=Sta.top();
                    ++i;
                    break;
                }
                case '-':{
                    flag=-Sta.top();
                    ++i;
                    break;
                }
                case '(':{
                    Sta.push(flag);
                    ++i;
                    break;
                }
                case ')':{
                    Sta.pop();
                    ++i;
                    break;
                }
                default:{
                    long num=0;
                    while(i<n&&s[i]>='0'&&s[i]<='9'){
                        num=num*10+s[i]-'0';
                        ++i;
                    }
                    ans+=flag*num;
                }
            }
        }
        return ans;
    }
};

227. 基本计算器 II

class Solution {
public:
    int calculate(string s) {
        int ans=0,x=0,n=s.size();
        char sign='+';
        stack<int> nums;
        for(int i=0;i<n;++i) {
            if(s[i]>='0') {
                x=x*10-'0'+s[i];
            }
            if((s[i]<'0'&&s[i]!=' ')||i==n-1) {
                if(sign=='+') {
                    nums.push(x);
                } else if(sign=='-') {
                    nums.push(-x);
                } else if(sign=='*'||sign=='/') {
                    int tmp=sign=='*'?nums.top()*x:nums.top()/x;
                    nums.pop();
                    nums.push(tmp);
                }
                sign=s[i];
                x=0;
            }
        }
        for(;!nums.empty();nums.pop()) {
            ans+=nums.top();
        }
        return ans;
    }
};

class Solution {
    public int calculate(String s) {
        int n=s.length();
        int x=0;
        int ans=0;
        Stack<Integer> st=new Stack<>();
        char sign='+';
        for(int i=0;i<n;++i) {
            if(Character.isDigit(s.charAt(i))) {
                x=x*10-'0'+s.charAt(i);
            }
            if(((!Character.isDigit(s.charAt(i)))&&s.charAt(i)!=' ')||i==n-1) {
                if(sign=='+') {
                    st.push(x);
                } else if(sign=='-') {
                    st.push(-x);
                } else if(sign=='*'||sign=='/') {
                    int tmp=sign=='*'?st.pop()*x:st.pop()/x;
                    st.push(tmp);
                }
                sign=s.charAt(i);
                x=0;
            }
        }
        for(;!st.empty();) {
            ans+=st.pop();
        }
        return ans;
    }
}

331. 验证二叉树的前序序列化

class Solution {
public:
    bool isValidSerialization(string preorder) {
        int cur=0,num=0,n=preorder.size();
        for(int i=0;i<n;++i) {
            if(isdigit(preorder[i])) {
                num=1;   
            } else if(preorder[i]==',') {
                cur+=num;
                if(cur<0&&i!=n-1)
                    return false;
            } else if(preorder[i]=='#'){
                num=-1;
            }
        }
        return cur+num==-1;
    }
};

class Solution {
    public boolean isValidSerialization(String preorder) {
        int n = preorder.length();
        int i = 0;
        Deque<Integer> stack = new LinkedList<Integer>();
        stack.push(1);
        while (i < n) {
            if (stack.isEmpty()) {
                return false;
            }
            if (preorder.charAt(i) == ',') {
                i++;
            } else if (preorder.charAt(i) == '#'){
                int top = stack.pop() - 1;
                if (top > 0) {
                    stack.push(top);
                }
                i++;
            } else {
                // 读一个数字
                while (i < n && preorder.charAt(i) != ',') {
                    i++;
                }
                int top = stack.pop() - 1;
                if (top > 0) {
                    stack.push(top);
                }
                stack.push(2);
            }
        }
        return stack.isEmpty();
    }
}

// 作者：LeetCode-Solution
// 链接：https://leetcode-cn.com/problems/verify-preorder-serialization-of-a-binary-tree/solution/yan-zheng-er-cha-shu-de-qian-xu-xu-lie-h-jghn/
// 来源：力扣（LeetCode）
// 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

705. 设计哈希集合

Official solution

class MyHashSet {
private:
    vector<list<int>> data;
    static const int base = 769;
    static int hash(int key) {
        return key % base;
    }
public:
    /** Initialize your data structure here. */
    MyHashSet(): data(base) {}

    void add(int key) {
        int h = hash(key);
        for (auto it = data[h].begin(); it != data[h].end(); it++) {
            if ((*it) == key) {
                return;
            }
        }
        data[h].push_back(key);
    }

    void remove(int key) {
        int h = hash(key);
        for (auto it = data[h].begin(); it != data[h].end(); it++) {
            if ((*it) == key) {
                data[h].erase(it);
                return;
            }
        }
    }

    /** Returns true if this set contains the specified element */
    bool contains(int key) {
        int h = hash(key);
        for (auto it = data[h].begin(); it != data[h].end(); it++) {
            if ((*it) == key) {
                return true;
            }
        }
        return false;
    }
};

// 作者：LeetCode-Solution
// 链接：https://leetcode-cn.com/problems/design-hashset/solution/she-ji-ha-xi-ji-he-by-leetcode-solution-xp4t/
// 来源：力扣（LeetCode）
// 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

/**
 * Your MyHashSet object will be instantiated and called as such:
 * MyHashSet* obj = new MyHashSet();
 * obj->add(key);
 * obj->remove(key);
 * bool param_3 = obj->contains(key);
 */

706. 设计哈希映射

Official solution

class MyHashMap {
private:
    vector<list<pair<int, int>>> data;
    static const int base = 769;
    static int hash(int key) {
        return key % base;
    }
public:
    /** Initialize your data structure here. */
    MyHashMap(): data(base) {}

    /** value will always be non-negative. */
    void put(int key, int value) {
        int h = hash(key);
        for (auto it = data[h].begin(); it != data[h].end(); it++) {
            if ((*it).first == key) {
                (*it).second = value;
                return;
            }
        }
        data[h].push_back(make_pair(key, value));
    }

    /** Returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key */
    int get(int key) {
        int h = hash(key);
        for (auto it = data[h].begin(); it != data[h].end(); it++) {
            if ((*it).first == key) {
                return (*it).second;
            }
        }
        return -1;
    }

    /** Removes the mapping of the specified value key if this map contains a mapping for the key */
    void remove(int key) {
        int h = hash(key);
        for (auto it = data[h].begin(); it != data[h].end(); it++) {
            if ((*it).first == key) {
                data[h].erase(it);
                return;
            }
        }
    }
};

// 作者：LeetCode-Solution
// 链接：https://leetcode-cn.com/problems/design-hashmap/solution/she-ji-ha-xi-ying-she-by-leetcode-soluti-klu9/
// 来源：力扣（LeetCode）
// 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

/**
 * Your MyHashMap object will be instantiated and called as such:
 * MyHashMap* obj = new MyHashMap();
 * obj->put(key,value);
 * int param_2 = obj->get(key);
 * obj->remove(key);
 */

54. 螺旋矩阵

收圈法

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        vector<int> ans;
        int m=matrix.size();
        int n=matrix[0].size();
        circle(matrix,0,0,m-1,n-1,ans);
        return ans;
    }
    void circle(vector<vector<int>> matrix,int x1,int y1,int x2,int y2,vector<int> &ans) {
        if(x2<x1||y2<y1)
            return;
        if(x1==x2) {
            for(int i=y1;i<=y2;++i) 
                ans.push_back(matrix[x1][i]);
            return;
        }

        if(y1==y2) {
            for(int i=x1;i<=x2;++i)
                ans.push_back(matrix[i][y1]);
            return;
        }

        for (int i = y1; i < y2; i++) 
            ans.push_back(matrix[x1][i]);
        for (int i = x1; i < x2; i++) 
            ans.push_back(matrix[i][y2]);
        for (int i = y2; i > y1; i--) 
            ans.push_back(matrix[x2][i]);
        for (int i = x2; i > x1; i--) 
            ans.push_back(matrix[i][y1]);

        circle(matrix,x1+1,y1+1,x2-1,y2-1,ans);
    }
};

贪吃蛇解法

class Solution {
public:
    const int dirs[4][9]={{0,1},{1,0},{0,-1},{-1,0}};
    const int flag=101;

    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        vector<int> ans;
        int m=matrix.size();
        int n=matrix[0].size();

        for(int x=0,y=0,d=0,i=0;i<m*n;++i) {
            ans.push_back(matrix[x][y]);
            matrix[x][y]=101;
            int nx=x+dirs[d][0];
            int ny=y+dirs[d][10];
            if(nx<0||ny<0||nx>=m||ny>=n||matrix[nx][ny]==flag) {
                d=(d+1)%4;
                nx=x+dirs[d][0];
                ny=y+dirs[d][11];
            }
            x=nx;
            y=ny;
        }

        return ans;
    }
};

2020/01

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        vector<int> ans;
        if(matrix.empty())  return ans;
        int u=0;
        int d=matrix.size()-1;
        int l=0;
        int r=matrix[0].size()-1;
        while(true)
        {
            for(int i=l;i<=r;++i)   
                ans.push_back(matrix[u][i]);
            if(++u>d)
                break;
            for(int i=u;i<=d;++i)
                ans.push_back(matrix[i][r]);
            if(--r<l)
                break;
            for(int i=r;i>=l;--i)
                ans.push_back(matrix[d][i]);
            if(--d<u)
                break;
            for(int i=d;i>=u;--i)
                ans.push_back(matrix[i][l]);
            if(++l>r)
                break;
        }
        return ans;
    }
};

59. 螺旋矩阵 II

模拟时主要注意边界，对一些特殊样例不要越界。

贪吃蛇解法

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<vector<int>> ans(n,vector<int>(n));
        int high=0,low=n-1,left=0,right=n-1;
        int number=0;
        while(number!=n*n) {
            for(int i=left;i<=right;++i) {
                ans[high][i]=++number;
            }
            for(int i=high+1;i<=low;++i) {
                ans[i][right]=++number;
            }
            for(int i=right-1;i>=left;--i) {
                ans[low][i]=++number;
            }
            for(int i=low-1;i>high;--i) {
                ans[i][left]=++number;
            }
            high++;
            low--;
            left++;
            right--;
        }
        return ans;
    }
};

2020/01

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        vector<int> r(n,0);
        vector<vector<int>> matrix(n,r);
        int x=0,y=-1;
        int high=0,low=0,left=0,right=0;
        int num=1;
        int tmpx,tmpy;
        while(true)
        {
            tmpx=x;
            tmpy=y;
            while(++y<n-right)
            {
                matrix[x][y]=num++;
            }
            high++;
            y--;
            while(++x<n-low)
            {
                matrix[x][y]=num++;
            }
            right++;
            x--;
            while(--y>=0+left)
            {
                matrix[x][y]=num++;
            }
            low++;
            y++;
            while(--x>=0+high)    
            {
                matrix[x][y]=num++;
            }
            left++;
            x++;
            if(x==tmpx && y==tmpy)
                break;
        }
        return matrix;
    }
};

115. 不同的子序列

My approach

class Solution {
public:
    int numDistinct(string s, string t) {
        int m=s.size();
        int n=t.size();
        if(m<n)
            return 0;
        //在字符串前添加一个空格，可以在f数组一列赋值为1，对结果不会产生影响。
        s=" "+s;
        t=" "+t;
        vector<vector<long>> f(m+1,vector<long>(n+1,0));
        for(int i=0;i<m+1;++i)
            f[i][0]=1;
        for(int i=1;i<=m;++i) {
            char sChar=s.at(i);
            for(int j=1;j<=n;++j) {
                char tChar=t.at(j);
                f[i][j]=f[i-1][j];
                if(sChar==tChar)
                    f[i][j]+=f[i-1][j-1];
            }
        }
        return f[m][n];
    }
};

Official solution

class Solution {
    public int numDistinct(String s, String t) {
        int m = s.length(), n = t.length();
        if (m < n) {
            return 0;
        }
        int[][] dp = new int[m + 1][n + 1];
        for (int i = 0; i <= m; i++) {
            dp[i][n] = 1;
        }
        for (int i = m - 1; i >= 0; i--) {
            char sChar = s.charAt(i);
            for (int j = n - 1; j >= 0; j--) {
                char tChar = t.charAt(j);
                if (sChar == tChar) {
                    dp[i][j] = dp[i + 1][j + 1] + dp[i + 1][j];
                } else {
                    dp[i][j] = dp[i + 1][j];
                }
            }
        }
        return dp[0][0];
    }
}

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/distinct-subsequences/solution/bu-tong-de-zi-xu-lie-by-leetcode-solutio-urw3/
来源：力扣（LeetCode）
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

92. 反转链表 II

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int left, int right) {
        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* l_node = dummy;
        right -= left;
        while(left-- > 1) 
            l_node = l_node->next;
        ListNode* a = l_node->next;
        ListNode* b = a->next;
        while(right-- > 0) {
            ListNode* tmp = b->next;
            b->next = a;
            a = b;
            b = tmp;
        }
        l_node->next->next = b;
        l_node->next = a;
        return dummy->next;
    }
};

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseBetween(ListNode head, int left, int right) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode l_node = dummy;
        right -= left;
        while(left-- > 1) {
            l_node = l_node.next;
        }
        ListNode a = l_node.next;
        ListNode b = a.next;
        while(right-- > 0) {
            ListNode tmp = b.next;
            b.next = a;
            a = b;
            b = tmp;
        }
        l_node.next.next = b;
        l_node.next = a;
        return dummy.next;
    }
}

1603. 设计停车系统

class ParkingSystem {
public:
    int b, m, s;
    ParkingSystem(int big, int medium, int small): b(big), m(medium), s(small) {}

    bool addCar(int carType) {
        if (carType == 1) {
            if (b > 0) {
                b--;
                return true;
            }
        } else if (carType == 2) {
            if (m > 0) {
                m--;
                return true;
            }
        } else if (carType == 3) {
            if (s > 0) {
                s--;
                return true;
            }
        }
        return false;
    }
};

// 作者：LeetCode-Solution
// 链接：https://leetcode-cn.com/problems/design-parking-system/solution/she-ji-ting-che-xi-tong-by-leetcode-solu-p52h/
// 来源：力扣（LeetCode）
// 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。
/**
 * Your ParkingSystem object will be instantiated and called as such:
 * ParkingSystem* obj = new ParkingSystem(big, medium, small);
 * bool param_1 = obj->addCar(carType);
 */

150. 逆波兰表达式求值

经典堆栈题目
该题主要需要处理的是字符串那块，顺带复习C++中switch的case无法识别字符串类型；C++中的stoi()函数可以将一个字符串（无论正负）都可以转换为相应的数值，而且stoi()会做个范围检查，一旦无法处理就会报错runtime error!。

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> Stack;
        for(int i = 0;i < tokens.size(); ++i) {
            if(tokens[i][0] >= '0' && tokens[i][0] <= '9' || tokens[i][0] == '-'&&tokens[i].size() > 1)
                Stack.push(stoi(tokens[i]));
            else {
                int b=Stack.top();
                Stack.pop();
                int a=Stack.top();
                Stack.pop();
                if(tokens[i] == "+") {
                    Stack.push(a+b);   
                }
                else if(tokens[i] == "-") {
                    Stack.push(a-b);
                }
                else if(tokens[i] == "*") {
                    Stack.push(a*b);
                }
                else if(tokens[i] == "/") {
                    Stack.push(a/b);
                }
            }
        }
        return Stack.top();
    }
};

官方Java版本
熟悉了Deque双端队列，equals()字符串等价函数，parseInt

class Solution {
    public int evalRPN(String[] tokens) {
        Deque<Integer> stack = new LinkedList<Integer>();
        int n = tokens.length;
        for (int i = 0; i < n; i++) {
            String token = tokens[i];
            if (isNumber(token)) {
                stack.push(Integer.parseInt(token));
            } else {
                int num2 = stack.pop();
                int num1 = stack.pop();
                switch (token) {
                    case "+":
                        stack.push(num1 + num2);
                        break;
                    case "-":
                        stack.push(num1 - num2);
                        break;
                    case "*":
                        stack.push(num1 * num2);
                        break;
                    case "/":
                        stack.push(num1 / num2);
                        break;
                    default:
                }
            }
        }
        return stack.pop();
    }

    public boolean isNumber(String token) {
        return !("+".equals(token) || "-".equals(token) || "*".equals(token) || "/".equals(token));
    }
}

// 作者：LeetCode-Solution
// 链接：https://leetcode-cn.com/problems/evaluate-reverse-polish-notation/solution/ni-bo-lan-biao-da-shi-qiu-zhi-by-leetcod-wue9/
// 来源：力扣（LeetCode）
// 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

73. 矩阵置零

使用两个标记数组

class Solution {
    public void setZeroes(int[][] matrix) {
        int m = matrix.length;
        int n = matrix[0].length;
        boolean[] row = new boolean[m];
        boolean[] col = new boolean[n];
        for(int i = 0; i < m; ++i) {
            for(int j = 0; j < n; ++j) {
                if(matrix[i][j] == 0) {
                    row[i] = col[j] = true;
                }
            }
        }
        for(int i = 0; i < m; ++i) {
            for(int j = 0; j < n; ++j) {
                if(row[i] || col[j]) {
                    matrix[i][j] = 0;
                }
            }
        }        
    }
}

使用两个标记变量

class Solution {
    public void setZeroes(int[][] matrix) {
        int m = matrix.length;
        int n = matrix[0].length;
        boolean flagRow = false;
        boolean flagCol = false;
        for(int j = 0; j < n;++j) 
            if(matrix[0][j] == 0)
                flagRow = true;
        for(int i = 0; i < m; ++i) 
            if(matrix[i][0] == 0)
                flagCol = true;
        for(int i = 1; i < m; ++i) {
            for(int j = 1; j < n; ++j) {
                if(matrix[i][j] == 0) 
                    matrix[i][0] = matrix[0][j] =0;
            }
        }
        for(int i = 1; i < m; ++i) {
            for(int j = 1; j < n; ++j) {
                if(matrix[i][0] == 0 || matrix[0][j] == 0) 
                    matrix[i][j] = 0;
            }
        }
        if(flagRow)
            for(int j = 0; j < n; ++j)
                matrix[0][j] = 0;
        if(flagCol)
            for(int i = 0; i < m; ++i)
                matrix[i][0] = 0;
    }
}

使用一个标记变量

class Solution {
    public void setZeroes(int[][] matrix) {
        int m = matrix.length;
        int n = matrix[0].length;
        boolean flagCol = false;
        for(int i = 0; i < m; ++i) {
            if(matrix[i][0] == 0)
                flagCol = true;
            for(int j = 1; j < n; ++j) {
                if(matrix[i][j] == 0) 
                    matrix[i][0] = matrix[0][j] = 0;
            }
        }
        for(int i = m - 1; i >= 0; --i) {
            for(int j = 1; j < n; ++j) {
                if(matrix[i][0] == 0 || matrix[0][j] == 0)
                    matrix[i][j]=0;
            }
            if(flagCol)
                matrix[i][0]=0;
        }
    }
}

191. 位1的个数

learn

位数检查

class Solution {
public:
    int hammingWeight(uint32_t n) {
        int ans = 0;
        for(int i = 0; i < 32; ++i) {
            ans += ((n >> i) & 1);
        }
        return ans;
    }
};

右移统计

class Solution {
public:
    int hammingWeight(uint32_t n) {
        int ans = 0;
        while(n) {
            ans += (n & 1);
            n >>=1;
        }
        return ans;
    }
};

341. 扁平化嵌套列表迭代器

月底回看重做

、暂无代码

456. 132 模式

宫水三叶的题解

class Solution {
public:
    bool find132pattern(vector<int>& nums) {
        stack<int> Sta;
        int k = INT_MIN;
        for(int i = nums.size() - 1; i >= 0; --i) {
            if(nums[i] < k)
                return true;
            while(!Sta.empty() && nums[i] > Sta.top()) {
                k = max(Sta.top(),k);
                Sta.pop();
            }
            Sta.push(nums[i]);
            // 下面是错误做法 
            // if(Sta.empty()) {
            //     Sta.push(nums[i]);
            // } else if(nums[i] > Sta.top()) {
            //     k = max(Sta.top(),k);
            //     Sta.pop();
            //     Sta.push(nums[i]);
            // } else if(k > nums[i]){
            //     flag = true;
            //     break;
            // }
        }
        return false;
    }
};

82. 删除排序链表中的重复元素 II

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        ListNode* dummy = new ListNode(-1);
        ListNode* tail = dummy;
        while(head) {
            if(head->next == nullptr || head->val != head->next->val) {
                tail->next = head;
                tail = head;
            }
            while(head->next != nullptr && head->val == head->next->val ) {
                head = head->next;
            }
            head = head->next;
        }
        tail->next = nullptr;
        return dummy->next;
    }
};

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode dummy = new ListNode(-1);
        ListNode tail = dummy;
        while(head != null) {
            if(head.next == null || head.val != head.next.val) {
                tail.next = head;
                tail = head;
            }
            while(head.next != null && head.val == head.next.val) {
                head = head.next;
            }
            head = head.next;
        }
        tail.next = null;
        return dummy.next;
    }
}

时间复杂度$O(n)$ 其中n为链表的长度
空间复杂度$O(1)$

83. 删除排序链表中的重复元素

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode dummy = new ListNode(-1);
        ListNode tail = dummy;
        while(head != null) {
            while(head.next != null && head.val == head.next.val) {
                head = head.next;
            }
            tail.next = head;
            tail = head;
            head = head.next;
        }
        tail.next = null;
        return dummy.next;
    }
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        ListNode* dummy = new ListNode(-1);
        ListNode* tail = dummy;
        while(head) {
            while(head->next != nullptr && head->val == head->next->val) {
                head = head->next;
            }
            tail->next = head;
            tail = head;
            head = head->next;
        }
        tail->next = nullptr;
        return dummy->next;
    }
};

61. 旋转链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* rotateRight(ListNode* head, int k) {
        if(k == 0 || head == nullptr || head->next == nullptr) 
            return head;
        ListNode* iter = head;
        int n = 1;
        while(iter->next) {
            iter = iter->next;
            n++;   
        }
        int m = k % n;
        if(m == 0) 
            return head;
        iter->next = head;
        for(int i = n - m; i > 0; --i) {
            iter = iter->next;
        }
        ListNode* ans = iter->next;
        iter->next = nullptr;
        return ans;
    }
};

173. 二叉搜索树迭代器

该题就是将二叉树的中序遍历过程进行了拆分（栈做法）：

不断遍历左子树，将左子树压入栈内
将最后压入栈内的元素pop出，作为答案加入中序遍历序列中
将pop的元素的右子树作为新的根节点，重复步骤1。

learn

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class BSTIterator {
private: 
    TreeNode* cur;
    stack<TreeNode*> stk;
public:
    BSTIterator(TreeNode* root) {
        cur = root;
    }

    int next() {
        while(cur) {
            stk.push(cur);
            cur = cur->left;
        }
        cur = stk.top();
        stk.pop();
        int ans = cur->val;
        cur = cur->right;
        return ans;
    }

    bool hasNext() {
        return cur || !stk.empty();
    }
};

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator* obj = new BSTIterator(root);
 * int param_1 = obj->next();
 * bool param_2 = obj->hasNext();
 */

190. 颠倒二进制位

class Solution {
public:
    uint32_t reverseBits(uint32_t n) {
        uint32_t ans = 0;
        for(int i = 1; i <= 32; ++i) {
            ans <<= 1;
            ans += 1&n;
            n >>= 1;
        }
        return ans;
    }
};

74. 搜索二维矩阵

因为测试数据都是规范的“长方形”存储数据，可以将该数据结构“拉长”为一个一维数组，再进行二分查找操作。

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int m = matrix.size();
        int n = matrix[0].size();
        int left = 0;
        int right = m * n -1;
        while(left <= right) {
            int mid = (right - left) / 2 + left;
            if(matrix[mid / n][mid % n] < target) 
                left = mid + 1;
            else if(matrix[mid / n][mid % n] > target)
                right = mid - 1;
            else 
                return true;
        }
        return false;
    }
};

90. 子集 II

learn

class Solution(object):
    def subsetsWithDup(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        nums.sort()
        n = len(nums)
        ans = set()
        cur = []
        i = 0
        while i < 1 << n:
            cur = []
            for j in range(n):
                if (i >> j) & 1:
                    cur.append(nums[j])
            ans.add(tuple(cur))
            i += 1
        return [list(x) for x in ans]

Last modification：October 6, 2023

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