Can anyone suggest me how to solve this problem(https://cses.fi/problemset/task/1087)
You are given a DNA sequence consisting of characters A, C, G, and T.
Your task is to find the shortest DNA sequence that is not a subsequence of the original sequence.
Input
The only input line contains a DNA sequence with n characters.
Output
Print the shortest DNA sequence that is not a subsequence of the original sequence. If there are several solutions, you may print any of them.
Constraints 1≤n≤1e6 Example
Input: ACGTACGT
Output: AAA
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you can modelled this problem as graph and can start thinking further