/**
 * 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) {
        if (root == null)
            return true;
        if ((root.left!=null && root.left.val>=root.val)||
            (root.right!=null && root.right.val<=root.val)) {
            return false;
        }
        return isValidBST(root.left) && isValidBST(root.right);
    }
}