Shared code

#include <bits/stdc++.h>
using namespace std;

const string fix = "ACGT";

constexpr int maxn = 1e6 + 4;
int mp[1 << 7];
char s[maxn];

void solve() {
    cin >> s;
    string ans;
    int matched = 0, n = strlen(s);
    for (int j = 0 ; j < n; ++j) {
        char i = s[j];
        int x = ++mp[i];
        if (x == 1) {
            ++matched;
        }
        if (matched == 4) {
            ans += i;
            for (char c: fix) mp[c] = 0;
            matched = 0;
        }
    }
    if (!matched) {
        ans += 'A';
    }
    else {
        for (char i: fix) {
            if (!mp[i]) {
                ans += i;
                break;
            }
        }
    }
    cout << ans << '\n';
}

signed main() {
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    solve();
    return 0;
}