~/boss / chest-row
Prefix sums
The Gold Towers
Stack the gold once, then answer any stretch with one subtraction.
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How to beat it
Walk the row once and keep a running total: tower k holds all the gold up to chest k, and tower 0 is the empty start. Gold from chest i to chest j is tower j minus tower i − 1. Subtract tower i instead and you lose chest i: that's the classic off-by-one.
Now the real boss
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