力扣

class Solution {
public:
    vector<string> commonChars(vector<string>& words) {
        vector<int> minfreq(26, INT_MAX);
        vector<int> freq(26);
        for (const string& word: words) {
            fill(freq.begin(), freq.end(), 0);
            for (char ch: word) {
                ++freq[ch - 'a'];
            }
            for (int i = 0; i < 26; ++i) {
                minfreq[i] = min(minfreq[i], freq[i]);
            }
        }

        vector<string> ans;
        for (int i = 0; i < 26; ++i) {
            for (int j = 0; j < minfreq[i]; ++j) {
                ans.emplace_back(1, i + 'a');
            }
        }
        return ans;
    }
};

class Solution {
public:
    vector<int> intersection(vector<int>& nums1, vector<int>& nums2) {
        unordered_set<int> set1, set2;
        for (auto& num : nums1) {
            set1.insert(num);
        }
        for (auto& num : nums2) {
            set2.insert(num);
        }
        return getIntersection(set1, set2);
    }

    vector<int> getIntersection(unordered_set<int>& set1, unordered_set<int>& set2) {
        if (set1.size() > set2.size()) {
            return getIntersection(set2, set1);
        }
        vector<int> intersection;
        for (auto& num : set1) {
            if (set2.count(num)) {
                intersection.push_back(num);
            }
        }
        return intersection;
    }
};

class Solution {
public:
    bool isHappy(int n) {
        unordered_set<int> set;
        int res = n;
        while (true) {
            res = square(res);
            if (res == 1) {
                return true;
            }
            if (set.find(res)!=set.end()) {//找到返回迭代器，失败返回end
                return false;
            }
            set.insert(res);
            
        }    
    }

    int square(int n) {
        int sum = 0, temp = 1;
        while (n != 0) {
            temp = n % 10;
            sum += temp * temp;
            n /= 10;
        }
        return sum;
    }
};

class Solution {
public:
    int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
        unordered_map<int, int> countAB;
        for (int u: A) {
            for (int v: B) {
                ++countAB[u + v];
            }
        }
        int ans = 0;
        for (int u: C) {
            for (int v: D) {
                if (countAB.count(-u - v)) {
                    ans += countAB[-u - v];
                }
            }
        }
        return ans;
    }
};

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> quadruplets;
        if (nums.size() < 4) {
            return quadruplets;
        }
        sort(nums.begin(), nums.end());
        int length = nums.size();
        for (int i = 0; i < length - 3; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            if ((long) nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {
                break;
            }
            if ((long) nums[i] + nums[length - 3] + nums[length - 2] + nums[length - 1] < target) {
                continue;
            }
            for (int j = i + 1; j < length - 2; j++) {
                if (j > i + 1 && nums[j] == nums[j - 1]) {
                    continue;
                }
                if ((long) nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {
                    break;
                }
                if ((long) nums[i] + nums[j] + nums[length - 2] + nums[length - 1] < target) {
                    continue;
                }
                int left = j + 1, right = length - 1;
                while (left < right) {
                    long sum = (long) nums[i] + nums[j] + nums[left] + nums[right];
                    if (sum == target) {
                        quadruplets.push_back({nums[i], nums[j], nums[left], nums[right]});
                        while (left < right && nums[left] == nums[left + 1]) {
                            left++;
                        }
                        left++;
                        while (left < right && nums[right] == nums[right - 1]) {
                            right--;
                        }
                        right--;
                    } else if (sum < target) {
                        left++;
                    } else {
                        right--;
                    }
                }
            }
        }
        return quadruplets;
    }
};

class Solution {
public:
    bool canConstruct(string ransomNote, string magazine) {
        if (ransomNote.size() > magazine.size()) {
            return false;
        }
        vector<int> cnt(26);
        for (auto & c : magazine) {
            cnt[c - 'a']++;
        }
        for (auto & c : ransomNote) {
            cnt[c - 'a']--;
            if (cnt[c - 'a'] < 0) {
                return false;
            }
        }
        return true;
    }
};

class Solution {
public:
	string convert(string s, int numRows) {

		if (numRows == 1) return s;

		vector<string> rows(min(numRows, int(s.size()))); // 防止s的长度小于行数
		int curRow = 0;
		bool goingDown = false;

		for (char c : s) {
			rows[curRow] += c;
			if (curRow == 0 || curRow == numRows - 1) {// 当前行curRow为0或numRows -1时，箭头发生反向转折
				goingDown = !goingDown;
			}
			curRow += goingDown ? 1 : -1;
		}

		string ret;
		for (string row : rows) {// 从上到下遍历行
			ret += row;
		}

		return ret;
	}
};

class Solution {
public:
    int reverse(int x) {
        int rev = 0;
        while (x != 0) {
            if (rev < INT_MIN / 10 || rev > INT_MAX / 10) {
                return 0;
            }
            int digit = x % 10;
            x /= 10;
            rev = rev * 10 + digit;
        }
        return rev;
    }
};

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map <int,int> map;
        for(int i = 0 ; i < nums.size(); i++){
            auto num = map.find(target - nums[i]);
            if(num != map.end()){
                return {i, num->second};
            }
            map[nums[i]] = i;
        }
        return {};
    }
};

class Solution {
public:
    bool isAnagram(string s, string t) {
        if (s.length() != t.length()) {
            return false;
        }
        vector<int> table(26, 0);
        for (auto& ch: s) {
            table[ch - 'a']++;
        }
        for (auto& ch: t) {
            table[ch - 'a']--;
            if (table[ch - 'a'] < 0) {
                return false;
            }
        }
        return true;
    }
};

class Solution {
public:
    void reverseString(vector<char>& s) {
        int n = s.size();
        for (int left = 0, right = n - 1; left < right; ++left, --right) {
            swap(s[left], s[right]);
        }
    }
};

class Solution {
public:
    string reverseStr(string s, int k) {
        int n = s.length();
        for (int i = 0; i < n; i += 2 * k) {
            reverse(s.begin() + i, s.begin() + min(i + k, n));
        }
        return s;
    }
};

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        int n = nums.size();
        sort(nums.begin(), nums.end());
        vector<vector<int>> ans;
        // 枚举 a
        for (int first = 0; first < n; ++first) {
            // 需要和上一次枚举的数不相同
            if (first > 0 && nums[first] == nums[first - 1]) {
                continue;
            }
            // c 对应的指针初始指向数组的最右端
            int third = n - 1;
            int target = -nums[first];
            // 枚举 b
            for (int second = first + 1; second < n; ++second) {
                // 需要和上一次枚举的数不相同
                if (second > first + 1 && nums[second] == nums[second - 1]) {
                    continue;
                }
                // 需要保证 b 的指针在 c 的指针的左侧
                while (second < third && nums[second] + nums[third] > target) {
                    --third;
                }
                // 如果指针重合，随着 b 后续的增加
                // 就不会有满足 a+b+c=0 并且 b<c 的 c 了，可以退出循环
                if (second == third) {
                    break;
                }
                if (nums[second] + nums[third] == target) {
                    ans.push_back({nums[first], nums[second], nums[third]});
                }
            }
        }
        return ans;
    }
};

class Solution {
public:
    string replaceSpace(string s) {
        int count = 0, len = s.size();
        // 统计空格数量
        for (char c : s) {
            if (c == ' ') count++;
        }
        // 修改 s 长度
        s.resize(len + 2 * count);
        // 倒序遍历修改
        for(int i = len - 1, j = s.size() - 1; i < j; i--, j--) {
            if (s[i] != ' ')
                s[j] = s[i];
            else {
                s[j - 2] = '%';
                s[j - 1] = '2';
                s[j] = '0';
                j -= 2;
            }
        }
        return s;
    }
};

//方法1---借助额外字符串
class Solution {
public:
    string reverseLeftWords(string s, int n) {
        string ans = s;
        int length = s.size();
        for(int i=0;i<length;i++)
        {
            ans[(i+length-n)%length] = s[i];
        }
        return ans;
    }
};

class Solution {
public:
    int strStr(string haystack, string needle) {
        int n = haystack.size(), m = needle.size();
        if (m == 0) {
            return 0;
        }
        vector<int> pi(m);
        for (int i = 1, j = 0; i < m; i++) {
            while (j > 0 && needle[i] != needle[j]) {
                j = pi[j - 1];
            }
            if (needle[i] == needle[j]) {
                j++;
            }
            pi[i] = j;
        }
        for (int i = 0, j = 0; i < n; i++) {
            while (j > 0 && haystack[i] != needle[j]) {
                j = pi[j - 1];
            }
            if (haystack[i] == needle[j]) {
                j++;
            }
            if (j == m) {
                return i - m + 1;
            }
        }
        return -1;
    }
};

class Solution {
public:
    void getNext (int* next, const string& s){
        next[0] = 0;
        int j = 0;
        for(int i = 1;i < s.size(); i++){
            while(j > 0 && s[i] != s[j]) {
                j = next[j - 1];
            }
            if(s[i] == s[j]) {
                j++;
            }
            next[i] = j;
        }
    }
    bool repeatedSubstringPattern (string s) {
        if (s.size() == 0) {
            return false;
        }
        int next[s.size()];
        getNext(next, s);
        int len = s.size();
        if (next[len - 1] != 0 && len % (len - (next[len - 1] )) == 0) {
            return true;
        }
        return false;
    }
};

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();
    }
};

class MyStack {
public:
    queue<int> queue1;
    queue<int> queue2;

    /** Initialize your data structure here. */
    MyStack() {

    }

    /** Push element x onto stack. */
    void push(int x) {
        queue2.push(x);
        while (!queue1.empty()) {
            queue2.push(queue1.front());
            queue1.pop();
        }
        swap(queue1, queue2);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int r = queue1.front();
        queue1.pop();
        return r;
    }
    
    /** Get the top element. */
    int top() {
        int r = queue1.front();
        return r;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return queue1.empty();
    }
};

class Solution {
public:
    string reverseWords(string s) {
        int left = 0, right = s.size() - 1;
        // 去掉字符串开头的空白字符
        while (left <= right && s[left] == ' ') ++left;

        // 去掉字符串末尾的空白字符
        while (left <= right && s[right] == ' ') --right;

        stack<string> d;
        string word;

        while (left <= right) {
            char c = s[left];
            if (word.size() && c == ' ') {
                // 将单词 push 到队列的头部
                d.push(word);
                word = "";
            }
            else if (c != ' ') {
                word += c;
            }
            ++left;
        }
        d.push(word);
        
        string ans;
        while (!d.empty()) {
            ans += d.top();
            d.pop();
            if (!d.empty()) ans += ' ';
        }
        return ans;
    }
};

class Solution {
public:
    bool isValid(string s) {
        // 设置unordered_map m，对应(为1，[为2，{为3，)为4，]为5，}为6
        stack<char> st;
        bool istrue=true;
        for(char c:s){
            int flag=m[c];
            if(flag>=1&&flag<=3) st.push(c);
            else if(!st.empty()&&m[st.top()]==flag-3) st.pop();
            else {istrue=false;break;}
        }
        if(!st.empty()) istrue=false;
        return istrue;
    }
};

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> stk;
        int n = tokens.size();
        for (int i = 0; i < n; i++) {
            string& token = tokens[i];
            if (isNumber(token)) {
                stk.push(atoi(token.c_str()));
            } else {
                int num2 = stk.top();
                stk.pop();
                int num1 = stk.top();
                stk.pop();
                switch (token[0]) {
                    case '+':
                        stk.push(num1 + num2);
                        break;
                    case '-':
                        stk.push(num1 - num2);
                        break;
                    case '*':
                        stk.push(num1 * num2);
                        break;
                    case '/':
                        stk.push(num1 / num2);
                        break;
                }
            }
        }
        return stk.top();
    }

    bool isNumber(string& token) {
        return !(token == "+" || token == "-" || token == "*" || token == "/");
    }
};

class Solution {
public:
    vector<int> maxSlidingWindow(vector<int>& nums, int k) {
        int n = nums.size();
        priority_queue<pair<int, int>> q;
        for (int i = 0; i < k; ++i) {
            q.emplace(nums[i], i);
        }
        vector<int> ans = {q.top().first};
        for (int i = k; i < n; ++i) {
            q.emplace(nums[i], i);
            while (q.top().second <= i - k) {
                q.pop();
            }
            ans.push_back(q.top().first);
        }
        return ans;
    }
};

class Solution {
public:


    static bool cmp(pair<int, int>&m, pair<int, int> & n){
        return m.second > n.second;
    }

    vector<int> topKFrequent(vector<int>& nums, int k) {
        unordered_map<int,int> map;
        for(int i = 0 ; i <nums.size(); i++){
            map[nums[i]]++;
        }
        priority_queue<pair<int,int>, vector<pair<int,int>>, decltype(&cmp)> q(cmp);

        for(auto& [num, count] : map){
            if(q.size() == k){
                if(q.top().second < count){
                    q.pop();
                    q.emplace(num, count);
                }
            }else{
                q.emplace(num, count);
            }
        }
        vector<int> res;
        while(!q.empty()){
            res.emplace_back(q.top().first);
            q.pop();
        }
        return res;
    }
};

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

class Solution {
public:
    int search(vector<int>& nums, int target) {
        int left = 0, right = nums.size() - 1;
        while(left <= right){
            int mid = (right - left) / 2 + left;
            int num = nums[mid];
            if (num == target) {
                return mid;
            } else if (num > target) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }
        return -1;
    }
};

class Solution {
public:
    int removeElement(vector<int>& nums, int val) {
        int n = nums.size();
        int left = 0;
        for (int right = 0; right < n; right++) {
            if (nums[right] != val) {
                nums[left] = nums[right];
                left++;
            }
        }
        return left;
    }
};

class Solution {
public:
    vector<int> sortedSquares(vector<int>& nums) {
        vector<int> ans;
        for (int num: nums) {
            ans.push_back(num * num);
        }
        sort(ans.begin(), ans.end());
        return ans;
    }
};

class Solution {
public:
    int minSubArrayLen(int s, vector<int>& nums) {
        int n = nums.size();
        if (n == 0) {
            return 0;
        }
        int ans = INT_MAX;
        int start = 0, end = 0;
        int sum = 0;
        while (end < n) {
            sum += nums[end];
            while (sum >= s) {
                ans = min(ans, end - start + 1);
                sum -= nums[start];
                start++;
            }
            end++;
        }
        return ans == INT_MAX ? 0 : ans;
    }
};

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        int maxNum = n * n;
        int curNum = 1;
        vector<vector<int>> matrix(n, vector<int>(n));
        int row = 0, column = 0;
        // 设置方向为右下左上vector<vector<int>> directions = {0,1}等
        int directionIndex = 0;
        while (curNum <= maxNum) {
            matrix[row][column] = curNum;
            curNum++;
            int nextRow = row + directions[directionIndex][0], nextColumn = column + directions[directionIndex][1];
            if (nextRow < 0 || nextRow >= n || nextColumn < 0 || nextColumn >= n || matrix[nextRow][nextColumn] != 0) {
                directionIndex = (directionIndex + 1) % 4;  // 顺时针旋转至下一个方向
            }
            row = row + directions[directionIndex][0];
            column = column + directions[directionIndex][1];
        }
        return matrix;
    }
};

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        // 哈希集合，记录每个字符是否出现过
        unordered_set<char> occ;
        int n = s.size();
        // 右指针，初始值为 -1，相当于我们在字符串的左边界的左侧，还没有开始移动
        int rk = -1, ans = 0;
        // 枚举左指针的位置，初始值隐性地表示为 -1
        for (int i = 0; i < n; ++i) {
            if (i != 0) {
                // 左指针向右移动一格，移除一个字符
                occ.erase(s[i - 1]);
            }
            while (rk + 1 < n && !occ.count(s[rk + 1])) {
                // 不断地移动右指针
                occ.insert(s[rk + 1]);
                ++rk;
            }
            // 第 i 到 rk 个字符是一个极长的无重复字符子串
            ans = max(ans, rk - i + 1);
        }
        return ans;
    }
};

class Solution {
public:
    int maxArea(vector<int>& height) {
        int i = 0, j = height.size() - 1, res = 0;
        while(i < j) {
            res = height[i] < height[j] ? 
                max(res, (j - i) * height[i++]): 
                max(res, (j - i) * height[j--]); 
        }
        return res;
    }
};

class Solution {
public:
    int trap(vector<int>& height) {
        if (height.size() <= 2) return 0;
        vector<int> maxLeft(height.size(), 0);
        vector<int> maxRight(height.size(), 0);
        int size = maxRight.size();

        // 记录每个柱子左边柱子最大高度
        maxLeft[0] = height[0];
        for (int i = 1; i < size; i++) {
            maxLeft[i] = max(height[i], maxLeft[i - 1]);
        }
        // 记录每个柱子右边柱子最大高度
        maxRight[size - 1] = height[size - 1];
        for (int i = size - 2; i >= 0; i--) {
            maxRight[i] = max(height[i], maxRight[i + 1]);
        }
        // 求和
        int sum = 0;
        for (int i = 0; i < size; i++) {
            int count = min(maxLeft[i], maxRight[i]) - height[i];
            if (count > 0) sum += count;
        }
        return sum;
    }
};

class Solution {
public:
    int largestRectangleArea(vector<int>& heights) {
        vector<int> minLeftIndex(heights.size());
        vector<int> minRightIndex(heights.size());
        int size = heights.size();

        // 记录每个柱子 左边第一个小于该柱子的下标
        minLeftIndex[0] = -1; // 注意这里初始化，防止下面while死循环
        for (int i = 1; i < size; i++) {
            int t = i - 1;
            // 这里不是用if，而是不断向左寻找的过程
            while (t >= 0 && heights[t] >= heights[i]) t = minLeftIndex[t];
            minLeftIndex[i] = t;
        }
        // 记录每个柱子 右边第一个小于该柱子的下标
        minRightIndex[size - 1] = size; // 注意这里初始化，防止下面while死循环
        for (int i = size - 2; i >= 0; i--) {
            int t = i + 1;
            // 这里不是用if，而是不断向右寻找的过程
            while (t < size && heights[t] >= heights[i]) t = minRightIndex[t];
            minRightIndex[i] = t;
        }
        // 求和
        int result = 0;
        for (int i = 0; i < size; i++) {
            int sum = heights[i] * (minRightIndex[i] - minLeftIndex[i] - 1);
            result = max(sum, result);
        }
        return result;
    }
};

class Solution {
public:
    string longestPalindrome(string s) {
        int n = s.size();
        if (n < 2) {
            return s;
        }

        int maxLen = 1;
        int begin = 0;
        // dp[i][j] 表示 s[i..j] 是否是回文串
        vector<vector<int>> dp(n, vector<int>(n));
        // 初始化：所有长度为 1 的子串都是回文串
        for (int i = 0; i < n; i++) {
            dp[i][i] = true;
        }
        // 递推开始
        // 先枚举子串长度
        for (int L = 2; L <= n; L++) {
            // 枚举左边界，左边界的上限设置可以宽松一些
            for (int i = 0; i < n; i++) {
                // 由 L 和 i 可以确定右边界，即 j - i + 1 = L 得
                int j = L + i - 1;
                // 如果右边界越界，就可以退出当前循环
                if (j >= n) {
                    break;
                }

                if (s[i] != s[j]) {
                    dp[i][j] = false;
                } else {
                    if (j - i < 3) {
                        dp[i][j] = true;
                    } else {
                        dp[i][j] = dp[i + 1][j - 1];
                    }
                }

                // 只要 dp[i][L] == true 成立，就表示子串 s[i..L] 是回文，此时记录回文长度和起始位置
                if (dp[i][j] && j - i + 1 > maxLen) {
                    maxLen = j - i + 1;
                    begin = i;
                }
            }
        }
        return s.substr(begin, maxLen);
    }
};

class MyLinedList(){
private:
    int size;
    ListNode* head;
public:
    MyLinedList(){
        this->size = 0;
        this->head = new ListNode(0);
    }
    int get(int index){
        if(index<0 || index >= size){
            return 0;
        }
        ListNode* cur = head;
        for(int cur_index = 0; cur_index <= index;cur_index++){
            cur = cur->next;
        }
        return cur->val;
    }
    int add_at_head(int val){
        add_at_index(0,val);
    }
    int add_at_tail(int val){
        add_at_index(size,val);
    }
    void add_at_index(int index, int val){
        if(index < 0 || index > size){
            return;
        }
        size++;
        ListNode* pre = head;
        ListNode* newone = new ListNode(val);

        for(int i = 0 ; i<index; i++){
            pre = pre->next;
        }
        newone->next = pre->next;
        pre->next = newone;
    }
    void delete_at_index(int index){
        if(index < 0 || index >= size){
            return;
        }
        ListNode* pre = head;
        for(int i = 0 ; i <index; i++){
            pre = pre->next;
        }
        ListNode* deleteone = pre->next;
        pre->next = pre->next->next;
        delete deleteone;
}
}

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *head = nullptr, *tail = nullptr;
        int carry = 0;
        while (l1 || l2) {
            int n1 = l1 ? l1->val: 0;
            int n2 = l2 ? l2->val: 0;
            int sum = n1 + n2 + carry;
            if (!head) {
                head = tail = new ListNode(sum % 10);
            } else {
                tail->next = new ListNode(sum % 10);
                tail = tail->next;
            }
            carry = sum / 10;
            if (l1) {
                l1 = l1->next;
            }
            if (l2) {
                l2 = l2->next;
            }
        }
        if (carry > 0) {
            tail->next = new ListNode(carry);
        }
        return head;
    }
};

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        if (head == nullptr) {
            return head;
        }
        head->next = removeElements(head->next, val);
        return head->val == val ? head->next : head;
    }
};

class Solution {
  public:
    bool isPalindrome(ListNode* head) {
      stack<int> rec;
      ListNode* temp = head;
      while(temp){
        rec.push(temp->val);
        temp = temp -> next;
      }
      while(!rec.empty() || head){
        if(head-> val == rec.top()){
          head = head->next;
          rec.pop(); 
        }
        else {
          return false;
        }
      }
      return true;
    }
}

  ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
    if(headA == nullptr || headB == nullptr){
      return nullptr;
    }
    ListNode* temp_a = headA;
    ListNode* temp_b = headB;
    while(temp_a != temp_b){
      if(temp_a == nullptr){
        temp_a = headB;
      }else{
        temp_a = temp_a ->next;
      }
      if(temp_b == nullptr){
        temp_b = headA;
      }else{
        temp_b = temp_b->next;
      }
    }
    return temp_a;
  }

ListNode* reverseList(ListNode* head) {
  if(head == nullptr || head->next == nullptr){
    return head;
  }
  ListNode* temp = reverseList(head->next);
  head->next->next =head;
  head->next = nullptr;
  return temp;
}

  ListNode* combine(ListNode* l1, ListNode* l2){
    ListNode* head = new ListNode(-1);
    ListNode* pre = head;
    while(l1 && l2){
      ListNode* temp1 = l1;
      ListNode* temp2 = l2;
      if(temp1->val >= temp2->val){
        pre->next = temp2;
        temp2 = temp2->next;
      }else{
        pre-> next = temp1;
        temp1 = temp1->next;
      }
      pre = pre->next;
    }
    if(l1 == nullptr){
      pre->next = l2;
    }
    if(l2 == nullptr){
      pre->next = l1;
    }
    return head->next;

  }

  bool hasCycle(ListNode *head) {
    ListNode* fast = head;
    ListNode* slow = head;
    if(head == nullptr || head->next == nullptr){
      return false;
    }
    while(fast != nullptr && fast->next != nullptr){
      slow = slow->next;
      fast = fast->next->next;
      if(fast == slow){
        return true;
      }
    }
    return false;
  }

  ListNode *detectCycle(ListNode *head) {
      ListNode* slow = head;        
      ListNode* fast = head;        
      while(fast && fast->next){           
          slow = slow->next;            
          fast = fast->next->next;            
          if(slow == fast){                
              fast = head;                
              while(fast != slow){                    
                  fast = fast->next;                    
                  slow = slow->next;                
              }                
              return fast;            
          }        
      }        
      return nullptr;    
  }

  ListNode* removeNthFromEnd(ListNode* head, int n) {
    ListNode* p = head;
    ListNode* q = head;
    while(n > 0){
      p = p->next;
      n--;
    }
    if(!p){
      return head->next;
    }
    while(p->next){
      p = p->next;
      q = q->next;
    }
    q->next = q->next->next;
    return q;
  }

class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (head == nullptr || head->next == nullptr) {
            return head;
        }
        ListNode* newHead = head->next;
        head->next = swapPairs(newHead->next);
        newHead->next = head;
        return newHead;
    }
};

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        stack<int> s1, s2;
        while (l1) {
            s1.push(l1 -> val);
            l1 = l1 -> next;
        }
        while (l2) {
            s2.push(l2 -> val);
            l2 = l2 -> next;
        }
        int carry = 0;
        ListNode* ans = nullptr;
        while (!s1.empty() || !s2.empty() || carry != 0) {
            int a = s1.empty() ? 0 : s1.top();
            int b = s2.empty() ? 0 : s2.top();
            if (!s1.empty()) s1.pop();
            if (!s2.empty()) s2.pop();
            int cur = a + b + carry;
            carry = cur / 10;
            cur %= 10;
            auto curnode = new ListNode(cur);
            curnode -> next = ans;
            ans = curnode;
        }
        return ans;
    }
};

  class Solution {
public:
    vector<ListNode*> splitListToParts(ListNode* head, int k) {
        int n = 0;
        ListNode *temp = head;
        while (temp != nullptr) {
            n++;
            temp = temp->next;
        }
        int quotient = n / k, remainder = n % k;

        vector<ListNode*> parts(k,nullptr);
        ListNode *curr = head;
        for (int i = 0; i < k && curr != nullptr; i++) {
            parts[i] = curr;
            int partSize = quotient + (i < remainder ? 1 : 0);
            for (int j = 1; j < partSize; j++) {
                curr = curr->next;
            }
            ListNode *next = curr->next;
            curr->next = nullptr;
            curr = next;
        }
        return parts;
    }
};

  ListNode* oddEvenList(ListNode* head) {
    if(!head || !head->next || !head->next->next){
      return head;
    }
    ListNode* odd = head;
    ListNode* even = head->next;
    ListNode* even_first = even;
    while(odd->next && even ->next){
      odd->next = even->next;
      odd = odd->next;
      even->next = odd->next;
      even = even->next;
    }
    odd->next = even_first;
    return head;

  }

class Solution {
private:
    
public:

    void reverse(ListNode* Node){
        if(Node == nullptr || Node->next == nullptr){
            return;
        }
        ListNode* next = Node->next;
        reverse(Node->next);
        next->next = Node;
    }
    ListNode *reverseBetween(ListNode *head, int left, int right) {
        ListNode* node = new ListNode(-1);
        node->next = head;
        ListNode* pre = node;
        for(int i = 0 ; i < left -1; i++){
            pre = pre->next;
        }
        ListNode* right1 = pre;

        for(int i = 0 ; i < right - left + 1; i++){
            right1 = right1 ->next;
        }

        ListNode* nextpart = right1->next;
        ListNode* changepart = pre->next;
        pre->next = nullptr;
        right1->next = nullptr;
        reverse(changepart);
        pre->next = right1;
        changepart->next = nextpart;
        
        return node->next;
    }
};

class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        if (!head) {
            return head;
        }

        ListNode* cur = head;
        while (cur->next) {
            if (cur->val == cur->next->val) {
                cur->next = cur->next->next;
            }
            else {
                cur = cur->next;
            }
        }

        return head;
    }
};

    vector<int> reversePrint(ListNode* head) {
      vector<int> res;
      ListNode* newhead = reverse(head);
      while(newhead){
        res.push_back(newhead->val);
        newhead = newhead->next;
      }
      return res;
    }
    ListNode* reverse(ListNode* head){
      if(!head || !head->next) return head;
      ListNode* temp = reverse(head->next);
      head->next->next = head;
      head->next = nullptr;
      return temp;
    }

class Solution {
public:
    void preorder(TreeNode *root, vector<int> &res) {
        if (root == nullptr) {
            return;
        }
        res.push_back(root->val);
        preorder(root->left, res);
        preorder(root->right, res);
    }

    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> res;
        preorder(root, res);
        return res;
    }
};

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector <vector <int>> ret;
        if (!root) {
            return ret;
        }

        queue <TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            int currentLevelSize = q.size();
            ret.push_back(vector <int> ());
            for (int i = 1; i <= currentLevelSize; ++i) {
                auto node = q.front(); q.pop();
                ret.back().push_back(node->val);
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
        }
        
        return ret;
    }
};

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        queue<TreeNode*> que;
        if (root != NULL) que.push(root);
        vector<vector<int>> result;
        while (!que.empty()) {
            int size = que.size();
            vector<int> vec;
            for (int i = 0; i < size; i++) {
                TreeNode* node = que.front();
                que.pop();
                vec.push_back(node->val);
                if (node->left) que.push(node->left);
                if (node->right) que.push(node->right);
            }
            result.push_back(vec);
        }
        reverse(result.begin(), result.end()); // 在这里反转一下数组即可
        return result;

    }
};

class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        queue<TreeNode*> que;
        if (root != NULL) que.push(root);
        vector<int> result;
        while (!que.empty()) {
            int size = que.size();
            for (int i = 0; i < size; i++) {
                TreeNode* node = que.front();
                que.pop();
                if (i == (size - 1)) result.push_back(node->val); // 将每一层的最后元素放入result数组中
                if (node->left) que.push(node->left);
                if (node->right) que.push(node->right);
            }
        }
        return result;
    }
};

class Solution {
public:
    vector<double> averageOfLevels(TreeNode* root) {
        queue<TreeNode*> que;
        if (root != NULL) que.push(root);
        vector<double> result;
        while (!que.empty()) {
            int size = que.size();
            double sum = 0; // 统计每一层的和
            for (int i = 0; i < size; i++) {
                TreeNode* node = que.front();
                que.pop();
                sum += node->val;
                if (node->left) que.push(node->left);
                if (node->right) que.push(node->right);
            }
            result.push_back(sum / size); // 将每一层均值放进结果集
        }
        return result;
    }
};

class Solution {
public:
    vector<vector<int>> levelOrder(Node* root) {
        queue<Node*> que;
        if (root != NULL) que.push(root);
        vector<vector<int>> result;
        while (!que.empty()) {
            int size = que.size();
            vector<int> vec;
            for (int i = 0; i < size; i++) {
                Node* node = que.front();
                que.pop();
                vec.push_back(node->val);
                for (int i = 0; i < node->children.size(); i++) { // 将节点孩子加入队列
                    if (node->children[i]) que.push(node->children[i]);
                }
            }
            result.push_back(vec);
        }
        return result;

    }
};