CF1789F Serval and Brain Power

定义一个字符串 $T$ 是好的，当且仅当存在字符串 $T'$ 和整数 $k(k\geq2)$，使得 $T$ 可以由 $k$ 个 $T'$ 首尾相接得到。 例如，字符串 $\texttt{gogogo}$ 就是好的，因为它可以由 $3$ 个字符串 $\texttt{go}$ 首尾相接得到；而 $\texttt{power}$ 就不是好的。 给定仅包含小写英文字母的字符串 $S$。 你需要求出 $S$ 最长的好的子序列（不一定连续）的长度，特别的如果 $S$ 没有好的子序列，答案为 $0$。

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For the first sample, all of the non-empty subsequences of buaa are listed below: - b - u - a (occurred twice, buaa and buaa) - bu - ba (occurred twice, buaa and buaa) - ua (occurred twice, buaa and buaa) - aa - bua (occurred twice, buaa and buaa ) - baa - uaa - buaa Since $ \texttt{aa} = \texttt{a} + \texttt{a} $ , aa is a powerful subsequence. It can be shown that aa is the only powerful subsequence among them. So the answer is $ 2 $ . For the second sample, the longest powerful subsequence is codcod from codeforcesround. So the answer is $ 6 $ .