var assert = require('assert');

​

function countUniValueSubtrees(root) {

  let count = 0;

  if (!root) {

    return 0;

  }

  isUniValHelper(root, count);

  return count;

}

​

function isUniValHelper(node, count) {

  // Base case: if both children are null, we know the node is a univalue, we can increment count and return true

  if (node.left === null && node.right === null) {

    count++;

    return true;

  }

​

  // Check isUniVal for each child search tree

  let isUniVal = true;

  if (node.left !== null) {

    isUniVal =

      isUniValHelper(node.left) && isUniVal && node.left.val === node.val;

  }

  if (node.right !== null) {

    isUniVal =

      isUniValHelper(node.right) && isUniVal && node.right.val === node.val;