Recursive Search Binary Tree C#

Iam trying to Findout if this program can check if a binary tree is BST or not,

Following is the Code:

public static bool isValidBST(Node root)
    {
        return isValid(root, root.Left.Value,
             root.Right.Value);
    }

    private static bool isValid(Node node, int MIN, int MAX)
    {
        // tree with no childres is BST
        if (node == null)
            return true;

        if (node.Value <= MIN || node.Value >= MAX)
            return false;

        return isValid(node.Left, MIN, node.Value) && isValid(node.Right, node.Value, MAX);    
    }

I Think there is somthing missing in my code for example i dont think this code would work on a tree with one root and just two nodes. can you guys help me to fix my implementation?

I also found this solution on stackoverflow

private static bool isValid(Node node, int MIN, int MAX)
    {
        // tree with no childres is BST
        if (node == null)
            return true;

        if (node.Value > MIN && node.Value < MAX
            && isValid(node.Left, MIN, Math.Min(node.Value, MAX))
            && isValid(node.Right, Math.Max(node.Value, MIN), MAX))
            return true;
        else
            return false;
    }

But this won't work for me eather!

this is how i tried the Code:

 public static void Main(string[] args)
    {
        Node n1 = new Node(1, null, null);
        Node n3 = new Node(3, null, null);
        Node n2 = new Node(2, n1, n3);

        Console.WriteLine(isValidBST(n2));
        Console.ReadLine();
    }

The result is False,while it should be True.