【LeetCode热题100（43/100）】验证二叉搜索树

题目地址：链接

思路： 中序遍历

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isValidBST = function(root) {
    const dfs = (root, left, right) => {
        if(!root) return true;
        return  root.val > left && root.val < right &&
                dfs(root.left, left, root.val) &&
                dfs(root.right, root.val, right);
    }
    return dfs(root, -Infinity, Infinity);
};