leetcode JAVA Validate Binary Search Tree 难度系数3 3.35

Question:

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.

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isValidBST(TreeNode root) {
return isValidBST(root,Integer.MIN_VALUE,Integer.MAX_VALUE);
}

private boolean isValidBST(TreeNode root, int lower, int upper) {
if(root==null) return true;
return root.val>lower&&root.val<upper&&isValidBST(root.left, lower, root.val)&&isValidBST(root.right, root.val, upper);
}
}