98. Validate Binary Search Tree

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