invert_binary_tree

Problem

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.