Editorial for Another Contest 3 Problem 3 - Lexicographically Largest Common Subsequence


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Submitting an official solution before solving the problem yourself is a bannable offence.

Lemma: The string is sorted in reverse alphabetic order.

Proof: By contradiction - if the string is not, take the first occurrence of an index i where s_i < s_{i+1}. Deleting character s_i results in a common subsequence that is lexicographically larger.

The solution follows directly from this - if a z is present in the answer, it must be the first letter, so find the first z in every string, and delete the prefix of each string up to and including that z. Repeat until some string doesn't contain a z. Repeat this process again with y, and loop through the letters in reverse alphabetic order.


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