/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {

        if(root == null) return true;

        if(root.right == null && root.left == null){
            return true;
        }

        return isValidHelper(root, Double.MAX_VALUE, Double.NEGATIVE_INFINITY);
    }

    public boolean isValidHelper(TreeNode root, double max, double min){

        if(root == null) return true;

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

        return isValidHelper(root.left, root.val, min) && isValidHelper(root.right, max, root.val);

    }

}