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Leetcode 672. Bulb Switcher II
Given n bulbs initially on and four toggle operations (flip all / evens / odds / positions 3k+1), determine how many distinct final configurations are reachable after exactly presses button presses. Key idea: the operations commute and the bulb pattern repeats every 6 positions, so the problem reduces to enumerating reachable bitmasks on a small prefix.
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