determine if a BST is valid

valid_tree.d

import std.stdio;

class Node
{
	int val;
	Node left, right;

	this(int val, Node left=null, Node right=null) {
		this.val = val;
		this.left = left;
		this.right = right;
	}
}

bool isValid(Node root) {
	return isValid(root, int.min, int.max);
}

bool isValid(Node root, int min, int max) {
	if (root is null) return true;
	if (root.val < min || root.val > max) return false;
	if(!isValid(root.left, min, root.val)) return false;
	if(!isValid(root.right, root.val, max)) return false;
	return true;
}

void main()
{
	auto root = new Node(10,
			new Node(5,
				new Node(3),
				new Node(7)),
			new Node(15,
				new Node(16),
				new Node(18)));
	writeln(isValid(root));
}