Day105_LeetCode98.java

/**
 * 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 helper(root, Long.MIN_VALUE , Long.MAX_VALUE);
    }

    public boolean helper(TreeNode root , long min , long max ){
        if(root==null){
            return true ;
        }

        if(root.val<=min || root.val>=max){
            return false ;
        }

        boolean leftAns = helper(root.left , min , root.val);
        boolean rightAns = helper(root.right , root.val , max );
        return leftAns && rightAns ;
    }
}