Maximum Depth of binary tree

Given the root of a binary tree, return its maximum depth.

A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Input: root = [3,9,20,null,null,15,7] Output: 3

Input: root = [1,null,2] Output: 2

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var maxDepth = function(root) {
  if (!root) {
      return 0;
  }

  const helper = (root) => {
      if (root === null) {
          return 0;
      }

      if (root.left === null && root.right === null) {
          return 1;
      }

      let left = helper(root.left);
      let right = helper(root.right);
      return Math.max(left, right) + 1;
  }

  return helper(root);
};