1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11     public boolean isValidBST(TreeNode root){
12         return isValidBST(root,Long.MIN_VALUE,Long.MAX_VALUE);
13     }
14 
15     public boolean isValidBST(TreeNode root,long minVal,long maxVal){
16         if(root==null) return true;
17         if(root.val>=maxVal|| root.val<=minVal) return false;
18         return isValidBST(root.left,minVal,root.val) && isValidBST(root.right,root.val,maxVal);
19     }
20 }