~/boss / empty-cage
Missing number (sum and XOR)
The Escape
One monster escaped. Find its cage with one number in your head.
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How to beat it
Cages 0 to n should add up to n(n+1)/2. Add up the monsters as they come back; the difference is the escaped one. XOR works the same way: pairs cancel, and only the missing number is left.
Now the real boss
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