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

        int? current = null;
        return IsValidBST(root, ref current);
    }

    public bool IsValidBST(TreeNode node, ref int? current)
    {
        if(node == null)
            return true;

        if(!IsValidBST(node.left, ref current))
            return false;

        if(current >= node.val)
            return false;

        current = node.val;

        if(!IsValidBST(node.right, ref current))
            return false;

        return true;
    }
}