1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root
    11.  * @return {number}
    12.  */
    13. var maxDepth = function(root) {
    14.   if(root === null) {
    15.     return 0
    16.   }
    18.   return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1
    19. };