226. Invert Binary Tree

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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.

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Time - O(n)
Space - O(h)
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