LC 104. Maximum Depth of Binary Tree

/**
 * 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
  } else {
    const left = maxDepth(root.left)
    const right = maxDepth(root.right)
    return Math.max(left, right) + 1
  }
};