CF335B Palindrome

Given a string $ s $ , determine if it contains any palindrome of length exactly $ 100 $ as a subsequence. If it has any, print any one of them. If it doesn't have any, print a palindrome that is a subsequence of $ s $ and is as long as possible.

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A subsequence of a string is a string that can be derived from it by deleting some characters without changing the order of the remaining characters. A palindrome is a string that reads the same forward or backward.