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 than the node's key.Both the left and right subtrees must also be binary search trees.Example 1:

Input:    2   / \  1   3Output: true

Example 2:

5   / \  1   4     / \    3   6Output: falseExplanation: 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.

难度：medium

题目：给定二叉树，判断其是否为二叉搜索树。

思路：递归，并记录当前结点的取值范围。

Runtime: 0 ms, faster than 100.00% of Java online submissions for Validate Binary Search Tree.

Memory Usage: 37.6 MB, less than 100.00% of Java online submissions for Validate Binary Search Tree.

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isValidBST(TreeNode root) {        if (null == root) {            return true;        }                return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);    }    public boolean isValidBST(TreeNode root, long minVal, long maxVal) {        if (null == root) {            return true;        }                if (root.val >= maxVal || root.val <= minVal) {            return false;        }                return isValidBST(root.left, minVal, root.val) && isValidBST(root.right, root.val, maxVal);    }}