Hello Guys I need hint and technique to learn to think in problems like this Problem Statement: 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 \le n \le 10^6

Example Input: ACGTACGT

Output: AAA

Problem Link: https://cses.fi/problemset/task/1087