### 226. Invert Binary Tree

Invert a binary tree.

Example:

Input:
4   /   \  2     7 / \   / \1   3 6   9
Output:
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 f*** off.

### Analyze

/** * 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) {  if (!root) return null
if (root.left || root.right) {    const tmp = root.left    root.left = root.right    root.right = tmp
invertTree(root.left)    invertTree(root.right)  }  return root};