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On maksay → Codeforces Beta Round #62, 15 years ago
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Let a and b be the number of accumulators with the biggest amount of energy and the smallest amount of energy. Initially a and b are 1. While a + b ≠ n move energy from the a accumulators with the most energy to b accumulators with the least energy such that
1) the energy remained in the accumulators with the most energy equals to the energy of accumulators with the most energy from the remaining ones (n - a - b). In this case a increases with 1.
2) the energy remained in the accumulators with the least energy equals to the energy of accumulators with the least energy from the remaining ones (n - a - b). In this case b increases with 1.

Here my source code (in C++): http://pastebin.com/m9Xc9rjy