/*
    For your reference:
    class BinaryTreeNode {
        Integer value;
        BinaryTreeNode left;
        BinaryTreeNode right;

        BinaryTreeNode(Integer value) {
            this.value = value;
            this.left = null;
            this.right = null;
        }
    }
    */

    private static int count = 0;

    static Integer find_single_value_trees(BinaryTreeNode root) {
        if (root == null) {
            return 0;
        }
        isUnival(root);
        return count;
    }

    // return true is the subtree starting from note is unival
    static boolean isUnival(BinaryTreeNode node) {

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

        boolean unival = true;
        if (node.left != null) {
            if (!isUnival(node.left)) { // make sure this is called if node.left is not null
                unival = false;
            }
            if (!node.value.equals(node.left.value)) {
                unival = false;
            }
        }

        if (node.right != null) {
            if (!isUnival(node.right)) {
                unival = false;
            }
            if (!node.value.equals(node.right.value)) {
                unival = false;
            }
        }
        if (unival) {
            count++;
        }

        return unival;
    }