Merge branch 'master' of github.com:youngyangyang04/leetcode-master

README.md

@@ -68,7 +68,7 @@
 
 我在题目讲解中统一使用C++，但你会发现下面几乎每篇题解都配有其他语言版本，Java、Python、Go、JavaScript等等，正是这些[热心小伙们](https://github.com/youngyangyang04/leetcode-master/graphs/contributors)的贡献的代码，当然我也会严格把控代码质量。 
 
-**所以也欢迎大家参与进来，完善题解的各个语言版本，拥抱开源，让更多小伙伴们收益**。
+**所以也欢迎大家参与进来，完善题解的各个语言版本，拥抱开源，让更多小伙伴们受益**。
 
 准备好了么，刷题攻略开始咯，go go go！
 

problems/0001.两数之和.md

@@ -152,25 +152,11 @@ class Solution:
     def twoSum(self, nums: List[int], target: int) -> List[int]:
         records = dict()
 
-        # 用枚举更方便，就不需要通过索引再去取当前位置的值
+        for index, value in enumerate(nums):
-        for idx, val in enumerate(nums):
+            if target - value in records:
-            if target - val not in records:
+                return [records[target- value], index]
-                records[val] = idx
+            records[value] = index
-            else:
+        return []
-                return [records[target - val], idx] # 如果存在就返回字典记录索引和当前索引
-```
-
-Python (v2):
-
-```python
-class Solution:
-    def twoSum(self, nums: List[int], target: int) -> List[int]:
-        rec = {}
-        for i in range(len(nums)):
-            rest = target - nums[i]
-            # Use get to get the index of the data, making use of one of the dictionary properties.
-            if rec.get(rest, None) is not None: return [rec[rest], i]
-            rec[nums[i]] = i
 ```
 
 Go：

problems/0015.三数之和.md

@@ -655,7 +655,7 @@ impl Solution {
 
 ```Rust
 // 双指针法
-use std::collections::HashSet;
+use std::cmp::Ordering;
 impl Solution {
     pub fn three_sum(nums: Vec<i32>) -> Vec<Vec<i32>> {
         let mut result: Vec<Vec<i32>> = Vec::new();
@@ -667,21 +667,20 @@ impl Solution {
             if i > 0 && nums[i] == nums[i - 1] { continue; }
             let (mut left, mut right) = (i + 1, len - 1);
             while left < right {
-                if nums[i] + nums[left] + nums[right] > 0 {
+                match (nums[i] + nums[left] + nums[right]).cmp(&0){
-                    right -= 1;
+		    Ordering::Equal =>{
-                    // 去重
-                    while left < right && nums[right] == nums[right + 1] { right -= 1; }
-                } else if nums[i] + nums[left] + nums[right] < 0 {
-                    left += 1;
-                    // 去重
-                    while left < right && nums[left] == nums[left - 1] { left += 1; }
-                } else {
 		        result.push(vec![nums[i], nums[left], nums[right]]);
-                    // 去重
+			left +=1;
-                    right -= 1;
+			right -=1;
+			while left < right && nums[left] == nums[left - 1]{
 			    left += 1;
-                    while left < right && nums[right] == nums[right + 1] { right -= 1; }
+			}
-                    while left < right && nums[left] == nums[left - 1] { left += 1; }
+			while left < right && nums[right] == nums[right+1]{
+			    right -= 1;
+			}
+		    }
+		    Ordering::Greater => right -= 1,
+		    Ordering::Less => left += 1,
 		}
             }
         }

problems/0018.四数之和.md

@@ -527,6 +527,7 @@ public class Solution
 
 Rust:
 ```Rust
+use std::cmp::Ordering;
 impl Solution {
     pub fn four_sum(nums: Vec<i32>, target: i32) -> Vec<Vec<i32>> {
         let mut result: Vec<Vec<i32>> = Vec::new();
@@ -545,21 +546,24 @@ impl Solution {
                 if i > k + 1 && nums[i] == nums[i - 1] { continue; }
                 let (mut left, mut right) = (i + 1, len - 1);
                 while left < right {
-                    if nums[k] + nums[i] > target - (nums[left] + nums[right]) {
+                    match (nums[k] + nums[i] + nums[left] + nums[right]).cmp(&target){
-                        right -= 1;
+		        Ordering::Equal => {
-                        // 去重
-                        while left < right && nums[right] == nums[right + 1] { right -= 1; }
-                    } else if nums[k] + nums[i] < target - (nums[left] + nums[right]) {
-                        left += 1;
-                        // 去重
-                        while left < right && nums[left] == nums[left - 1] { left += 1; }
-                    } else {
 			    result.push(vec![nums[k], nums[i], nums[left], nums[right]]);
-                        // 去重
-                        while left < right && nums[right] == nums[right - 1] { right -= 1; }
-                        while left < right && nums[left] == nums[left + 1] { left += 1; }
 			    left += 1;
 			    right -= 1;
+			    while left < right && nums[left] == nums[left - 1]{
+			        left += 1;
+			    }
+			    while left < right && nums[right] == nums[right + 1]{
+			        right -= 1;
+			    }
+			}
+			Ordering::Less => {
+			    left +=1;
+			},
+			Ordering::Greater => {
+			    right -= 1;
+			}
 		    }
                 }
             }

problems/0019.删除链表的倒数第N个节点.md

@@ -364,6 +364,40 @@ impl Solution {
         dummy_head.next
     }
 }
+```
+C语言
+```c
+/**c语言单链表的定义
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+struct ListNode* removeNthFromEnd(struct ListNode* head, int n) {
+    //定义虚拟头节点dummy 并初始化使其指向head
+    struct ListNode* dummy = malloc(sizeof(struct ListNode));
+    dummy->val = 0;
+    dummy->next = head;
+    //定义 fast slow 双指针
+    struct ListNode* fast = head;
+    struct ListNode* slow = dummy;
+
+    for (int i = 0; i < n; ++i) {
+        fast = fast->next;
+    }
+    while (fast) {
+        fast = fast->next;
+        slow = slow->next;
+    }
+    slow->next = slow->next->next;//删除倒数第n个节点
+    head = dummy->next;
+    free(dummy);//删除虚拟节点dummy
+    return head;
+}
+
+
+
 ```
 
 <p align="center">

problems/0028.实现strStr.md

@@ -1299,6 +1299,53 @@ impl Solution {
 }
 ```
 
+> 前缀表统一减一
+
+```rust
+impl Solution {
+    pub fn get_next(next_len: usize, s: &Vec<char>) -> Vec<i32> {
+        let mut next = vec![-1; next_len];
+        let mut j = -1;
+        for i in 1..s.len() {
+            while j >= 0 && s[(j + 1) as usize] != s[i] {
+                j = next[j as usize];
+            }
+            if s[i] == s[(j + 1) as usize] {
+                j += 1;
+            }
+            next[i] = j;
+        }
+        next
+    }
+    pub fn str_str(haystack: String, needle: String) -> i32 {
+        if needle.is_empty() {
+            return 0;
+        }
+        if haystack.len() < needle.len() {
+            return -1;
+        }
+        let (haystack_chars, needle_chars) = (
+            haystack.chars().collect::<Vec<char>>(),
+            needle.chars().collect::<Vec<char>>(),
+        );
+        let mut j = -1;
+        let next = Self::get_next(needle.len(), &needle_chars);
+        for (i, v) in haystack_chars.into_iter().enumerate() {
+            while j >= 0 && v != needle_chars[(j + 1) as usize] {
+                j = next[j as usize];
+            }
+            if v == needle_chars[(j + 1) as usize] {
+                j += 1;
+            }
+            if j == needle_chars.len() as i32 - 1 {
+                return (i - needle_chars.len() + 1) as i32;
+            }
+        }
+        -1
+    }
+}
+```
+
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>

problems/0052.N皇后II.md

@@ -256,7 +256,54 @@ int totalNQueens(int n){
     return answer;
 }
 ```
-
+Java
+```java
+class Solution {
+    int count = 0;
+    public int totalNQueens(int n) {
+        char[][] board = new char[n][n];
+        for (char[] chars : board) {
+            Arrays.fill(chars, '.');
+        }
+        backTrack(n, 0, board);
+        return count;
+    }
+    private void backTrack(int n, int row, char[][] board) {
+        if (row == n) {
+            count++;
+            return;
+        }
+        for (int col = 0; col < n; col++) {
+            if (isValid(row, col, n, board)) {
+                board[row][col] = 'Q';
+                backTrack(n, row + 1, board);
+                board[row][col] = '.';
+            }
+        }
+    }
+    private boolean isValid(int row, int col, int n, char[][] board) {
+        // 检查列
+        for (int i = 0; i < row; ++i) {
+            if (board[i][col] == 'Q') {
+                return false;
+            }
+        }
+        // 检查45度对角线
+        for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--) {
+            if (board[i][j] == 'Q') {
+                return false;
+            }
+        }
+        // 检查135度对角线
+        for (int i = row - 1, j = col + 1; i >= 0 && j <= n - 1; i--, j++) {
+            if (board[i][j] == 'Q') {
+                return false;
+            }
+        }
+        return true;
+    }
+}
+```
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>

problems/0084.柱状图中最大的矩形.md

@@ -409,7 +409,44 @@ class Solution:
 
 
 ```
-*****
+
+Go:
+
+> 单调栈
+
+```go
+func largestRectangleArea(heights []int) int {
+ // 声明max并初始化为0
+ max := 0
+ // 使用切片实现栈
+ stack := make([]int, 0)
+ // 数组头部加入0
+ heights = append([]int{0}, heights...)
+ // 数组尾部加入0
+ heights = append(heights, 0)
+ // 初始化栈，序号从0开始
+ stack = append(stack, 0)
+ for i := 1; i < len(heights); i++ {
+  // 结束循环条件为：当即将入栈元素>top元素，也就是形成非单调递增的趋势
+  for heights[stack[len(stack)-1]] > heights[i] {
+   // mid 是top
+   mid := stack[len(stack)-1]
+   // 出栈
+   stack = stack[0 : len(stack)-1]
+   // left是top的下一位元素，i是将要入栈的元素
+   left := stack[len(stack)-1]
+   // 高度x宽度
+   tmp := heights[mid] * (i - left - 1)
+   if tmp > max {
+    max = tmp
+   }
+  }
+  stack = append(stack, i)
+ }
+ return max
+}
+
+```
 
 JavaScript:
 ```javascript

problems/0090.子集II.md

@@ -262,6 +262,8 @@ class Solution:
 ```
 
 ### Python3
+
+不使用used数组
 ```python3
 class Solution:
     def subsetsWithDup(self, nums: List[int]) -> List[List[int]]:
@@ -288,6 +290,28 @@ class Solution:
         return res
 ```
 
+使用used数组
+```python3
+class Solution:
+    def subsetsWithDup(self, nums: List[int]) -> List[List[int]]:
+        result = []
+        path = []
+        nums.sort()
+        used = [0] * len(nums)
+        def backtrack(nums, startIdx):
+            result.append(path[:])
+            for i in range(startIdx, len(nums)):
+                if i > startIdx and nums[i] == nums[i-1] and used[i-1] == 0:
+                    continue
+                used[i] = 1
+                path.append(nums[i])
+                backtrack(nums, i+1)
+                path.pop()
+                used[i] = 0
+        backtrack(nums, 0)
+        return result
+```
+
 ### Go 
 
 ```Go
@@ -526,7 +550,7 @@ func subsetsWithDup(_ nums: [Int]) -> [[Int]] {
 
 ### Scala
 
-不使用userd数组:
+不使用used数组:
 
 ```scala
 object Solution {

problems/0121.买卖股票的最佳时机.md

@@ -464,6 +464,47 @@ function maxProfit(prices: number[]): number {
 };
 ```
 
+C#：
+
+> 贪心法
+
+```csharp
+public class Solution
+{
+    public int MaxProfit(int[] prices)
+    {
+        int min = Int32.MaxValue;
+        int res = 0;
+        for (int i = 0; i < prices.Length; i++)
+        {
+            min = Math.Min(prices[i], min);
+            res = Math.Max(prices[i] - min, res);
+        }
+        return res;
+    }
+}
+```
+
+> 动态规划
+
+```csharp
+public class Solution
+{
+	public int MaxProfit(int[] prices)
+    {
+        int[] dp = new int[2];
+        int size = prices.Length;
+        (dp[0], dp[1]) = (-prices[0], 0);
+        for (int i = 0; i < size; i++)
+        {
+            dp[0] = Math.Max(dp[0], -prices[i]);
+            dp[1] = Math.Max(dp[1], dp[0]+prices[i]);
+        }
+        return dp[1];
+    }
+}
+```
+
 
 
 
@@ -471,3 +512,4 @@ function maxProfit(prices: number[]): number {
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>
 </a>
+

problems/0122.买卖股票的最佳时机II（动态规划）.md

@@ -169,8 +169,6 @@ class Solution {
 }
 ```
 
-
-
 Python：
 
 > 版本一：
@@ -253,8 +251,6 @@ func max(a,b int)int{
 }
 ```
 
-
-
 Javascript：
 ```javascript
 // 方法一：动态规划（dp 数组）
@@ -331,9 +327,47 @@ function maxProfit(prices: number[]): number {
 };
 ```
 
+C#：
+
+> 贪心法
+
+```csharp
+public class Solution
+{
+    public int MaxProfit(int[] prices)
+    {
+        int res = 0;
+        for (int i = 1; i < prices.Length; i++)
+            res += Math.Max(0, prices[i] - prices[i-1]);
+        return res;
+    }
+}
+```
+
+> 动态规划
+
+```csharp
+public class Solution
+{
+   public int MaxProfit(int[] prices)
+    {
+        int[] dp = new int[2];
+        dp[0] = -prices[0];
+
+        for (int i = 1; i < prices.Length; i++)
+        {
+            dp[0] = dp[0]>dp[1] - prices[i]?dp[0]:dp[1] - prices[i];
+            dp[1] = dp[1] > dp[0] + prices[i] ? dp[1] : dp[0] + prices[i];
+        }
+        return dp[1];
+    }
+}
+```
+
 
 
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>
 </a>
+

problems/0127.单词接龙.md

@@ -31,7 +31,7 @@
 
 # 思路
 
-以示例1为例，从这个图中可以看出 hit 到 cog的路线，不止一条，有三条，两条是最短的长度为5，一条长度为6。
+以示例1为例，从这个图中可以看出 hit 到 cog的路线，不止一条，有三条，一条是最短的长度为5，两条长度为6。
 
 ![](https://code-thinking-1253855093.file.myqcloud.com/pics/20210827175432.png)
 

problems/0216.组合总和III.md

@@ -416,24 +416,31 @@ func backTree(n,k,startIndex int,track *[]int,result *[][]int){
  * @return {number[][]}
  */
 var combinationSum3 = function(k, n) {
-    const backtrack = (start) => {
+    let res = [];
-        const l = path.length;
+    let path = [];
-        if (l === k) {
+    let sum = 0;
-            const sum = path.reduce((a, b) => a + b);
+    const dfs = (path,index) => {
-            if (sum === n) {
+        // 剪枝操作
+        if (sum > n){
+            return 
+        }
+        if (path.length == k) {
+            if(sum == n){
                 res.push([...path]);
+                return 
             }
-            return;
         }
-        for (let i = start; i <= 9 - (k - l) + 1; i++) {
+        for (let i = index; i <= 9 - (k-path.length) + 1;i++) {
             path.push(i);
-            backtrack(i + 1);
+            sum = sum + i;
-            path.pop();
+            index += 1;
+            dfs(path,index);
+            sum -= i
+            path.pop()
         }
     }
-    let res = [], path = [];
+    dfs(path,1);
-    backtrack(1);
+    return res
-    return res;
 };
 ```
 

problems/0235.二叉搜索树的最近公共祖先.md

@@ -328,13 +328,11 @@ var lowestCommonAncestor = function(root, p, q) {
     }
     if(root.val>p.val&&root.val>q.val) {
         // 向左子树查询
-        let left = lowestCommonAncestor(root.left,p,q);
+         return root.left = lowestCommonAncestor(root.left,p,q);
-        return left !== null&&left;
     }
     if(root.val<p.val&&root.val<q.val) {
         // 向右子树查询
-        let right = lowestCommonAncestor(root.right,p,q);
+        return root.right = lowestCommonAncestor(root.right,p,q);
-        return right !== null&&right;
     }
     return root;
 };

problems/0239.滑动窗口最大值.md

@@ -88,7 +88,7 @@ public:
 
 对于窗口里的元素{2, 3, 5, 1 ,4}，单调队列里只维护{5, 4} 就够了，保持单调队列里单调递减，此时队列出口元素就是窗口里最大元素。
 
-此时大家应该怀疑单调队列里维护着{5, 4} 怎么配合窗口经行滑动呢？
+此时大家应该怀疑单调队列里维护着{5, 4} 怎么配合窗口进行滑动呢？
 
 设计单调队列的时候，pop，和push操作要保持如下规则：
 
@@ -297,15 +297,18 @@ class Solution {
 
 Python：
 ```python
+from collections import deque
+
+
 class MyQueue: #单调队列（从大到小
     def __init__(self):
-        self.queue = [] #使用list来实现单调队列
+        self.queue = deque() #这里需要使用deque实现单调队列，直接使用list会超时
     
     #每次弹出的时候，比较当前要弹出的数值是否等于队列出口元素的数值，如果相等则弹出。
     #同时pop之前判断队列当前是否为空。
     def pop(self, value):
         if self.queue and value == self.queue[0]:
-            self.queue.pop(0)#list.pop()时间复杂度为O(n),这里可以使用collections.deque()
+            self.queue.popleft()#list.pop()时间复杂度为O(n),这里需要使用collections.deque()
             
     #如果push的数值大于入口元素的数值，那么就将队列后端的数值弹出，直到push的数值小于等于队列入口元素的数值为止。
     #这样就保持了队列里的数值是单调从大到小的了。

problems/0332.重新安排行程.md

@@ -355,16 +355,18 @@ class Solution:
         tickets_dict = defaultdict(list)
         for item in tickets:
             tickets_dict[item[0]].append(item[1])
+	# 给每一个机场的到达机场排序，小的在前面，在回溯里首先被pop(0）出去
+	# 这样最先找的的path就是排序最小的答案，直接返回
+	for airport in tickets_dict: tickets_dict[airport].sort()
         '''
         tickets_dict里面的内容是这样的
-         {'JFK': ['SFO', 'ATL'], 'SFO': ['ATL'], 'ATL': ['JFK', 'SFO']})
+         {'JFK': ['ATL', 'SFO'], 'SFO': ['ATL'], 'ATL': ['JFK', 'SFO']})
         '''
         path = ["JFK"]
         def backtracking(start_point):
             # 终止条件
             if len(path) == len(tickets) + 1:
                 return True
-            tickets_dict[start_point].sort()
             for _ in tickets_dict[start_point]:
                 #必须及时删除，避免出现死循环
                 end_point = tickets_dict[start_point].pop(0)

problems/0349.两个数组的交集.md

@@ -91,7 +91,7 @@ public:
 
 对应C++代码如下：
 
-```c++
+```CPP
 class Solution {
 public:
     vector<int> intersection(vector<int>& nums1, vector<int>& nums2) {
@@ -310,6 +310,22 @@ impl Solution {
 }
 ```
 
+解法2:
+
+```rust
+use std::collections::HashSet;
+impl Solution {
+    pub fn intersection(nums1: Vec<i32>, nums2: Vec<i32>) -> Vec<i32> {
+        nums1
+            .into_iter()
+            .collect::<HashSet<_>>()
+            .intersection(&nums2.into_iter().collect::<HashSet<_>>())
+            .copied()
+            .collect()
+    }
+}
+```
+
 C:
 ```C
 int* intersection1(int* nums1, int nums1Size, int* nums2, int nums2Size, int* returnSize){

problems/0450.删除二叉搜索树中的节点.md

@@ -344,6 +344,48 @@ class Solution:
         return root
 ```
 
+**迭代法**
+```python
+class Solution:
+    def deleteNode(self, root: Optional[TreeNode], key: int) -> Optional[TreeNode]:
+        # 找到节点后分两步，1. 把节点的左子树和右子树连起来，2. 把右子树跟父节点连起来
+        # root is None
+        if not root: return root
+        p = root
+        last = None
+        while p:
+            if p.val==key:
+                # 1. connect left to right
+                # right is not None -> left is None | left is not None
+                if p.right:
+                    if p.left:
+                        node = p.right
+                        while node.left:
+                            node = node.left
+                        node.left = p.left
+                    right = p.right
+                else:
+                # right is None -> right=left
+                    right = p.left
+                # 2. connect right to last
+                if last==None:
+                    root = right
+                elif last.val>key:
+                    last.left = right
+                else:
+                    last.right = right
+                # 3. return
+                break
+            else:
+                # Update last and continue
+                last = p
+                if p.val>key:
+                    p = p.left
+                else:
+                    p = p.right
+        return root
+```
+
 ## Go
 ```Go
 // 递归版本

problems/0459.重复的子字符串.md

@@ -615,6 +615,37 @@ impl Solution {
 }
 ```
 
+>前缀表统一减一
+
+```rust
+impl Solution {
+    pub fn get_next(next_len: usize, s: &Vec<char>) -> Vec<i32> {
+        let mut next = vec![-1; next_len];
+        let mut j = -1;
+        for i in 1..s.len() {
+            while j >= 0 && s[i] != s[(j + 1) as usize] {
+                j = next[j as usize];
+            }
+            if s[i] == s[(j + 1) as usize] {
+                j += 1;
+            }
+            next[i] = j;
+        }
+        next
+    }
+    pub fn repeated_substring_pattern(s: String) -> bool {
+        let s_chars = s.chars().collect::<Vec<char>>();
+        let next = Self::get_next(s_chars.len(), &s_chars);
+        if next[s_chars.len() - 1] >= 0
+            && s_chars.len() % (s_chars.len() - (next[s_chars.len() - 1] + 1) as usize) == 0
+        {
+            return true;
+        }
+        false
+    }
+}
+```
+
 
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">

problems/0518.零钱兑换II.md

@@ -7,7 +7,7 @@
 
 ## 518. 零钱兑换 II
 
-[力扣题目链接](https://leetcode.cn/problems/coin-change-2/)
+[力扣题目链接](https://leetcode.cn/problems/coin-change-ii/)
 
 难度：中等
 

problems/0541.反转字符串II.md

@@ -399,19 +399,42 @@ object Solution {
   }
 }
 ```
-版本二: 首先利用sliding每隔k个进行分割，随后转换为数组，再使用zipWithIndex添加每个数组的索引，紧接着利用map做变换，如果索引%2==0则说明需要翻转，否则原封不动，最后再转换为String
+版本二: 首先利用grouped每隔k个进行分割，再使用zipWithIndex添加每个数组的索引，紧接着利用map做变换，如果索引%2==0则说明需要翻转，否则原封不动，最后再转换为String
 ```scala
 object Solution {
   def reverseStr(s: String, k: Int): String = {
-    s.sliding(k, k)
+    // s = "abcdefg", k = 2
-      .toArray
+    s.grouped(k) // Iterator ["ab", "cd", "ef", "g"]
-      .zipWithIndex
+      .zipWithIndex // Iterator [("ab", 0), ("cd", 1), ("ef", 2), ("g", 3)]
-      .map(v => if (v._2 % 2 == 0) v._1.reverse else v._1)
+      .map {
+        case (subStr, index) => 
+          if (index % 2 == 0) subStr.reverse else subStr
+      }
       .mkString
   }
 }
 ```
 
+版本三: (递归)
+```scala
+import scala.annotation.tailrec
+
+object Solution {
+  def reverseStr(s: String, k: Int): String = {
+    @tailrec // 这个函数已经优化成了尾递归
+    def reverse(s: String, needToReverse: Boolean, history: String): String = {
+      // 截取前k个字符（判断是否翻转）
+      val subStr = if (needToReverse) s.take(k).reverse else s.take(k)
+      // 如果字符串长度小于k，返回结果
+      // 否则，对于剩余字符串进行同样的操作
+      if (s.length < k) history + subStr
+      else reverse(s.drop(k), !needToReverse, history + subStr)
+    }
+    reverse(s, true, "")
+  }
+}
+```
+
 Rust:
 
 ```Rust

problems/0701.二叉搜索树中的插入操作.md

@@ -330,6 +330,26 @@ class Solution:
         return root
 ```
 
+**递归法** - 无返回值 有注释 不用Helper function
+```python
+class Solution:
+    def insertIntoBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
+        if not root:    # for root==None
+            return TreeNode(val)
+        if root.val<val:
+            if root.right==None:    # find the parent
+                root.right = TreeNode(val)
+            else:   # not found, keep searching
+                self.insertIntoBST(root.right, val)
+        if root.val>val:
+            if root.left==None: # found the parent
+                root.left = TreeNode(val)
+            else:   # not found, keep searching
+                self.insertIntoBST(root.left, val)
+        # return the final tree
+        return root
+```
+
 **迭代法**  
 与无返回值的递归函数的思路大体一致 
 ```python
@@ -337,16 +357,15 @@ class Solution:
     def insertIntoBST(self, root: TreeNode, val: int) -> TreeNode:
         if not root: 
             return TreeNode(val)
-        parent = None
+        parent = None # 此步可以省略
         cur = root
 
         # 用while循环不断地找新节点的parent
         while cur: 
+            parent = cur  # 首先保存当前非空节点作为下一次迭代的父节点
             if cur.val < val: 
-                parent = cur
                 cur = cur.right
             elif cur.val > val: 
-                parent = cur
                 cur = cur.left
 
         # 运行到这意味着已经跳出上面的while循环, 

problems/1002.查找常用字符.md

@@ -451,6 +451,42 @@ object Solution {
   }
 }
 ```
+
+Rust:
+
+```rust
+impl Solution {
+    pub fn common_chars(words: Vec<String>) -> Vec<String> {
+        if words.is_empty() {
+            return vec![];
+        }
+        let mut res = vec![];
+        let mut hash = vec![0; 26];
+        for i in words[0].bytes() {
+            hash[(i - b'a') as usize] += 1;
+        }
+        for i in words.iter().skip(1) {
+            let mut other_hash_str = vec![0; 26];
+            for j in i.bytes() {
+                other_hash_str[(j - b'a') as usize] += 1;
+            }
+            for k in 0..26 {
+                hash[k] = hash[k].min(other_hash_str[k]);
+            }
+        }
+
+        for (i, v) in hash.iter_mut().enumerate() {
+            while *v > 0 {
+                res.push(((i as u8 + b'a') as char).to_string());
+                *v -= 1;
+            }
+        }
+
+        res
+    }
+}
+```
+
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>

problems/1221.分割平衡字符串.md

@@ -117,7 +117,7 @@ class Solution:
                 diff -= 1
             else:
                 diff += 1
-            if tilt == 0:
+            if diff == 0:
                 ans += 1
         return ans
 ```

problems/栈与队列理论基础.md

@@ -59,7 +59,7 @@ C++标准库是有多个版本的，要知道我们使用的STL是哪个版本
 ![栈与队列理论3](https://img-blog.csdnimg.cn/20210104235459376.png)
 
 
-**我们常用的SGI STL，如果没有指定底层实现的话，默认是以deque为缺省情况下栈的低层结构。**
+**我们常用的SGI STL，如果没有指定底层实现的话，默认是以deque为缺省情况下栈的底层结构。**
 
 deque是一个双向队列，只要封住一段，只开通另一端就可以实现栈的逻辑了。