Today, finally for the first time in history, I got a test case to hack the problem 2155D - Batteries.

But the contest had already ended :(

The test case is actually easy:

(n = 40, a = 4) and the active positions are (1,14,27,40). It should be solved in under (400) operations, but everyone who wrote code that checks by iterating over the gaps between the batteries will get the result in approximately (403)–(404) queries.

One day I'll also hack, today I got the motivation.