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.