Phone Interview

Got a call through initial screening at the career fair at my university. The interviewer asked one coding question:

Given a string of 'a's, 'b's, and 'c's, which can appear in any order and any number of times, reduce the string to a string in which every combination of two different characters is replaced by the third one (e.g., 'ab' becomes 'c', 'bc' becomes 'a', 'cb' becomes 'a', etc.). Continue this process until no further reductions are possible.

Note that in-place modification and creating a new string will yield separate answers. You may choose any approach.

Example: 'aabbcc' becomes 'bb' ('aabbcc' -> 'acac' -> 'bb').

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