P50279: Control C302E

#include <iostream>
using namespace std;


char complement(char c) {
    if (c == 'G') return 'C';
    else if (c == 'T') return 'A';
    else if (c == 'A') return 'T';
    else return 'G';
}

int main() {
    char c;
    int es = 0;
    bool seq = false;
        while (not seq && cin >> c) {
            if (es == 0) {
                if (c == 'T') ++es;
            }
            else if (es == 1) {
                if (c == 'A') ++es;
                else if (c != 'A' or c != 'T') --es;
            }
            else if (es == 2) {
                if (c == 'G') ++es;
                else if (c == 'T') --es;
                else es = 0;
            }
            if (es == 3) seq = true;
        }
        if (seq) {
            while (cin >> c) cout << complement(c);
        }
    cout << endl;
}