[pull] master from youngyangyang04:master

problems/0019.删除链表的倒数第N个节点.md

@@ -22,10 +22,12 @@
 
 输入：head = [1,2,3,4,5], n = 2
 输出：[1,2,3,5]
+
 示例 2：
 
 输入：head = [1], n = 1
 输出：[]
+
 示例 3：
 
 输入：head = [1,2], n = 1
@@ -192,16 +194,18 @@ func removeNthFromEnd(head *ListNode, n int) *ListNode {
  * @param {number} n
  * @return {ListNode}
  */
-var removeNthFromEnd = function(head, n) {
-    let ret = new ListNode(0, head),
-        slow = fast = ret;
-    while(n--) fast = fast.next;
-    while (fast.next !== null) {
-        fast = fast.next; 
-        slow = slow.next
-    };
-    slow.next = slow.next.next;
-    return ret.next;
+var removeNthFromEnd = function (head, n) {
+  // 创建哨兵节点，简化解题逻辑
+  let dummyHead = new ListNode(0, head);
+  let fast = dummyHead;
+  let slow = dummyHead;
+  while (n--) fast = fast.next;
+  while (fast.next !== null) {
+    slow = slow.next;
+    fast = fast.next;
+  }
+  slow.next = slow.next.next;
+  return dummyHead.next;
 };
 ```
 ### TypeScript:

problems/0077.组合.md

@@ -469,28 +469,58 @@ func dfs(n int, k int, start int) {
 ```
 
 ### Javascript
+未剪枝：
+
+```js
+var combine = function (n, k) {
+  // 回溯法
+  let result = [],
+    path = [];
+  let backtracking = (_n, _k, startIndex) => {
+    // 终止条件
+    if (path.length === _k) {
+      result.push(path.slice());
+      return;
+    }
+    // 循环本层集合元素
+    for (let i = startIndex; i <= _n; i++) {
+      path.push(i);
+      //   递归
+      backtracking(_n, _k, i + 1);
+      //   回溯操作
+      path.pop();
+    }
+  };
+  backtracking(n, k, 1);
+  return result;
+};
+```
 
 剪枝：
 
 ```javascript
-let result = []
-let path = []
-var combine = function(n, k) {
-  result = []
-  combineHelper(n, k, 1)
-  return result
+var combine = function (n, k) {
+  // 回溯法
+  let result = [],
+    path = [];
+  let backtracking = (_n, _k, startIndex) => {
+    // 终止条件
+    if (path.length === _k) {
+      result.push(path.slice());
+      return;
+    }
+    // 循环本层集合元素
+    for (let i = startIndex; i <= _n - (_k - path.length) + 1; i++) {
+      path.push(i);
+      //   递归
+      backtracking(_n, _k, i + 1);
+      //   回溯操作
+      path.pop();
+    }
+  };
+  backtracking(n, k, 1);
+  return result;
 };
-const combineHelper = (n, k, startIndex) => {
-  if (path.length === k) {
-    result.push([...path])
-    return
-  }
-  for (let i = startIndex; i <= n - (k - path.length) + 1; ++i) {
-    path.push(i)
-    combineHelper(n, k, i + 1)
-    path.pop()
-  }
-}
 ```
 
 ### TypeScript

problems/0102.二叉树的层序遍历.md

@@ -692,27 +692,29 @@ func levelOrderBottom(root *TreeNode) [][]int {
 #### Javascript:
 
 ```javascript
-var levelOrderBottom = function(root) {
-    let res = [], queue = [];
-    queue.push(root);
-    while(queue.length && root!==null) {
-        // 存放当前层级节点数组
-        let curLevel = [];
-        // 计算当前层级节点数量
-        let length = queue.length;
-        while(length--) {
-            let node = queue.shift();
-            // 把当前层节点存入curLevel数组
-            curLevel.push(node.val);
-            // 把下一层级的左右节点存入queue队列
-            node.left && queue.push(node.left);
-            node.right && queue.push(node.right);
-        }
-	// 从数组前头插入值，避免最后反转数组，减少运算时间
-        res.unshift(curLevel);
-    }
-    return res;
+var levelOrderBottom = function (root) {
+  let res = [],
+    queue = [];
+  queue.push(root);
+  while (queue.length && root !== null) {
+    // 存放当前层级节点数组
+    let curLevel = [];
+    // 计算当前层级节点数量
+    let length = queue.length;
+    while (length--) {
+      let node = queue.shift();
+      // 把当前层节点存入curLevel数组
+      curLevel.push(node.val);
+      // 把下一层级的左右节点存入queue队列
+      node.left && queue.push(node.left);
+      node.right && queue.push(node.right);
+    }
+    // 从数组前头插入值，避免最后反转数组，减少运算时间
+    res.unshift(curLevel);
+  }
+  return res;
 };
+
 ```
 
 #### TypeScript:
@@ -1140,7 +1142,7 @@ impl Solution {
 
 ### 思路
 
-本题就是层序遍历的时候把一层求个总和在取一个均值。
+本题就是层序遍历的时候把一层求个总和再取一个均值。
 
 C++代码:
 
@@ -1295,26 +1297,26 @@ func averageOfLevels(root *TreeNode) []float64 {
 
 ```javascript
 var averageOfLevels = function(root) {
-    //层级平均值
-    let res = [], queue = [];
-    queue.push(root);
-
-    while(queue.length && root!==null) {
-        //每一层节点个数
-        let length = queue.length;
-        //sum记录每一层的和
-        let sum = 0;
-        for(let i=0; i < length; i++) {
-            let node = queue.shift();
-            sum += node.val;
-            node.left && queue.push(node.left);
-            node.right && queue.push(node.right);
-        }
-        //每一层的平均值存入数组res
-        res.push(sum/length);
-    }
-
-    return res;
+  let res = [],
+    queue = [];
+  queue.push(root);
+  while (queue.length) {
+    // 每一层节点个数;
+    let lengthLevel = queue.length,
+      len = queue.length,
+      //   sum记录每一层的和;
+      sum = 0;
+    while (lengthLevel--) {
+      const node = queue.shift();
+      sum += node.val;
+      //   队列存放下一层节点
+      node.left && queue.push(node.left);
+      node.right && queue.push(node.right);
+    }
+    // 求平均值
+    res.push(sum / len);
+  }
+  return res;
 };
 ```
 
@@ -1925,26 +1927,28 @@ func max(x, y int) int {
 #### Javascript：
 
 ```javascript
-var largestValues = function(root) {
-    //使用层序遍历
-    let res = [], queue = [];
-    queue.push(root);
-
-    while(root !== null && queue.length) {
-        //设置max初始值就是队列的第一个元素
-        let max = queue[0].val;
-        let length = queue.length;
-        while(length--) {
-            let node = queue.shift();
-            max = max > node.val ? max : node.val;
-            node.left && queue.push(node.left);
-            node.right && queue.push(node.right);
-        }
-        //把每一层的最大值放到res数组
-        res.push(max);
-    }
-
+var largestValues = function (root) {
+  let res = [],
+    queue = [];
+  queue.push(root);
+  if (root === null) {
     return res;
+  }
+  while (queue.length) {
+    let lengthLevel = queue.length,
+      // 初始值设为负无穷大
+      max = -Infinity;
+    while (lengthLevel--) {
+      const node = queue.shift();
+      //   在当前层中找到最大值
+      max = Math.max(max, node.val);
+      // 找到下一层的节点
+      node.left && queue.push(node.left);
+      node.right && queue.push(node.right);
+    }
+    res.push(max);
+  }
+  return res;
 };
 ```
 
@@ -2805,21 +2809,23 @@ func maxDepth(root *TreeNode) int {
  * @param {TreeNode} root
  * @return {number}
  */
-var maxDepth = function(root) {
-    // 最大的深度就是二叉树的层数
-    if (root === null) return 0;
-    let queue = [root];
-    let height = 0;
-    while (queue.length) {
-        let n = queue.length;
-        height++;
-        for (let i=0; i<n; i++) {
-            let node = queue.shift();
-            node.left && queue.push(node.left);
-            node.right && queue.push(node.right);
-        }
+var maxDepth = function (root) {
+  // 二叉树的 最大深度 是指从根节点到最远叶子节点的最长路径上的节点数。
+  let max = 0,
+    queue = [root];
+  if (root === null) {
+    return max;
+  }
+  while (queue.length) {
+    max++;
+    let length = queue.length;
+    while (length--) {
+      let node = queue.shift();
+      node.left && queue.push(node.left);
+      node.right && queue.push(node.right);
     }
-    return height;
+  }
+  return max;
 };
 ```
 

problems/0209.长度最小的子数组.md

@@ -270,22 +270,21 @@ var minSubArrayLen = function(target, nums) {
 
 ```typescript
 function minSubArrayLen(target: number, nums: number[]): number {
-    let left: number = 0, right: number = 0;
-    let res: number = nums.length + 1;
-    let sum: number = 0;
-    while (right < nums.length) {
-        sum += nums[right];
-        if (sum >= target) {
-            // 不断移动左指针，直到不能再缩小为止
-            while (sum - nums[left] >= target) {
-                sum -= nums[left++];
-            }
-            res = Math.min(res, right - left + 1);
-        }
-        right++;
+  let left: number = 0,
+    res: number = Infinity,
+    subLen: number = 0,
+    sum: number = 0;
+  for (let right: number = 0; right < nums.length; right++) {
+    sum += nums[right];
+    while (sum >= target) {
+      subLen = right - left + 1;
+      res = Math.min(res, subLen);
+      sum -= nums[left];
+      left++;
     }
-    return res === nums.length + 1 ? 0 : res;
-};
+  }
+  return res === Infinity ? 0 : res;
+}
 ```
 
 ### Swift: