~/boss / coin-combos
Combination sum (backtracking)
The Exact Change
Coins of 3, 5 and 7. Find every way to pay exactly 15.
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How to beat it
Add a coin and subtract it from what's left. Exactly 0? Record the pile. Below 0? Dead end: step back. Never add a coin smaller than the last one, so 3+5 and 5+3 can't both show up.
Now the real boss
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