maximum_depth_of_binary_tree.js

function TreeNode(val) {
  this.val = val;
  this.left = this.right = null;
}

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

  let left_height = maxDepth(root.left);
  let right_height = maxDepth(root.right);

  return Math.max(left_height, right_height) + 1;
};