Write a function that returns whether a given binary tree is a valid binary search tree.

gistfile1.java

/**
 * Returns {@code true} if the node is the root of a vaild binary
 * search tree.
 */
public static boolean isBinarySearchTree(Node<T extends Comparable> node) {
    if (node == null) {
        return true;
    }

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

    if (node.left != null && node.left.value.compareTo(node.value) > 0) {
        return false;
    }

    if (node.right != null && node.right.value.compareTo(node.value) < 0) {
        return false;
    }

    return isBinarySearchTree(node.left) && isBinarySearchTree(node.right);
}