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.

给定一个字符串 $s$，判断它是否包含长度恰好为 100 的回文作为子序列。如果存在，输出其中任意一个；否则，输出 $s$ 的一个尽可能长的回文子序列。

The only line of the input contains one string s of length n (1 ≤ n ≤ 5·104) containing only lowercase English letters.

输入仅包含一行，该行有一个长度为 $n$（$1 ≤ n ≤ 5·10^4$）的字符串 $s$，且 $s$ 仅由小写英文字母组成。

If s contains a palindrome of length exactly 100 as a subsequence, print any palindrome of length 100 which is a subsequence of s. If s doesn't contain any palindromes of length exactly 100, print a palindrome that is a subsequence of s and is as long as possible.

If there exists multiple answers, you are allowed to print any of them.

如果字符串 $s$ 中存在一个长度恰好为 100 的回文子序列，则输出 $s$ 的任意一个长度为 100 的回文子序列；否则，输出 $s$ 的一个尽可能长的回文子序列。

若存在多个合法答案，输出其中任意一个即可。

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.

字符串的子序列是指通过删除原字符串中的一些字符（不改变剩余字符的相对顺序）而得到的字符串。回文串是指正读和反读都相同的字符串。