98. Validate Binary Search Tree（js）

98. Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater thanthe node's key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

Input:
    2
   / \
  1   3
Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
             is 5 but its right child's value is 4.
题意：判断一棵树是否是合法的二叉搜索树
代码如下：

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
// 中序遍历解法
var isValidBST = function(root) {
    if(!root) return true;
    let vals=[];
    inorder(root,vals);
    for(let i=0;i<vals.length-1;i++){
        if(vals[i]>=vals[i+1]) return false;
    }
    return true;
};
var inorder=function(root,vals){
    if(!root) return ;
    inorder(root.left,vals);
    vals.push(root.val);
    inorder(root.right,vals);
}