[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/

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做出优秀的产品不代表代码优秀，反之亦然。

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function(root) {
    swap(root);
    return root;

    function swap(node){
        if(node !== null){
            var tmp = node.left;
            node.left = node.right;
            node.right = tmp;
            swap(node.left);
            swap(node.right);
        }
    }
};