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.