91. invert a binary tree

Can you invert a binary tree and get an offer from Google?

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.

— Max Howell (@mxcl) June 10, 2015

Inverting a node means swapping its left child and right child. You need to apply this to all nodes. As following figure illustrates.