[LeetCode][JavaScript]Invert Binary Tree

Invert Binary Tree

Invert a binary tree.

     4
   /     2     7
 / \   / 1   3 6   9

to

     4
   /     7     2
 / \   / 9   6 3   1

Trivia:
This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

https://leetcode.com/problems/invert-binary-tree/

 1 /**
 2  * Definition for a binary tree node.
 3  * function TreeNode(val) {
 4  *     this.val = val;
 5  *     this.left = this.right = null;
 6  * }
 7  */
 8 /**
 9  * @param {TreeNode} root
10  * @return {TreeNode}
11  */
12 var invertTree = function(root) {
13     swap(root);
14     return root;
15
16     function swap(node){
17         if(node !== null){
18             var tmp = node.left;
19             node.left = node.right;
20             node.right = tmp;
21             swap(node.left);
22             swap(node.right);
23         }
24     }
25 };