226. Invert Binary Tree

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.

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