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Commit 867dc16

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docs: 新增No98、No104题解
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‎leetcode刷题/README.md

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- [No.48 旋转图像](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No48_rotate.md)
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- [No.66 加一](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No66_plus-one.md)
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- [No.70 爬楼梯](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No70_climb-stairs.md)
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- [No.98 验证二叉搜索树](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No98_is-valid-BST.md)
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- [No.104 二叉树的最大深度](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No104_max-depth.md)
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- [No.122 买卖股票的最佳时机 II](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No122_max-profit.md)
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- [No.125 验证回文串](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No125_is-palindrome.md)
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- [No.136 只出现一次的数字](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No136_single-number.md)

‎leetcode刷题/note/No104_max-depth.md

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# No.104 二叉树的最大深度
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难度:`easy`
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给定一个二叉树,找出其最大深度。
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二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。
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说明: 叶子节点是指没有子节点的节点。
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## 示例
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示例:
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给定二叉树 [3,9,20,null,null,15,7],
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```
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3
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/ \
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9 20
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/ \
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15 7
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返回它的最大深度 3 。
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```
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## 解题思路
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使用递归的思路,分别先计算左右子树的高度,然后返回最大值即可。
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代码如下:
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```javascript
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/**
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* Definition for a binary tree node.
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* function TreeNode(val) {
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* this.val = val;
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* this.left = this.right = null;
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* }
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*/
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/**
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* @param {TreeNode} root
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* @return {number}
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*/
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var maxDepth = function(root) {
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if(!root) return 0
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let left = maxDepth(root.left)
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let right = maxDepth(root.right)
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return Math.max(left,right) + 1
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};
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```
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**解题思路二:**
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如果使用迭代的方法,需要借助额外的栈空间。
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# No.98 验证二叉搜索树
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难度: `middle`
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给定一个二叉树,判断其是否是一个有效的二叉搜索树。
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假设一个二叉搜索树具有如下特征:
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- 节点的左子树只包含小于当前节点的数。
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- 节点的右子树只包含大于当前节点的数。
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- 所有左子树和右子树自身必须也是二叉搜索树。
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## 示例
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示例 1:
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```
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输入:
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2
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/ \
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1 3
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输出: true
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```
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示例 2:
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```
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输入:
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5
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/ \
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1 4
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/ \
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3 6
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输出: false
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解释: 输入为: [5,1,4,null,null,3,6]。
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根节点的值为 5 ,但是其右子节点值为 4 。
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```
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## 解题思路
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代码:
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```javascript
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/**
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* Definition for a binary tree node.
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* function TreeNode(val) {
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* this.val = val;
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* this.left = this.right = null;
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* }
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*/
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/**
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* @param {TreeNode} root
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* @return {boolean}
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*/
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var isValidBST = function(root) {
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return helper(root, null, null);
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};
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var helper = function(node, lower, upper) {
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if (node == null) return true;
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let val = node.val;
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if (lower != null && val <= lower) return false;
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if (upper != null && val >= upper) return false;
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if (! helper(node.right, val, upper)) return false;
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if (! helper(node.left, lower, val)) return false;
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return true;
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}
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```

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