~/boss / magic-mirror
Invert a binary tree (recursion)
The Mirror
Flip the tree into its reflection. One swap per node, in the order the recursion calls them.
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How to beat it
At each node, swap its left and right child. Then do the same to the left subtree, then the right one. A leaf has nothing to flip: its call just returns. The call stack remembers who is still waiting.
Now the real boss
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