/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solution { public boolean isValidBST(TreeNode root) { return isValidBST(root,null,null); } public boolean isValidBST(TreeNode root,TreeNode min,TreeNode max){ if(root == null){ return true; } if(min != null && root.val<=min.val) return false; if(max != null && root.val>=max.val) return false; return isValidBST(root.left,min,root)&&isValidBST(root.right,root,max); } }