华为机试：DNA序列

#include <iostream>
#include <string>
#include <map>
#include <algorithm>
using namespace std;

int main() {
    string DNA;
    cin >> DNA;
    int n;
    cin >> n;
    int len = DNA.length();
    double GC_Ratio = 0;
    string ans;
    for (int i = 0; i <= len - n; i++) {
        string tmp = DNA.substr(i, n);
        double temp = ((double)count(tmp.begin(), tmp.end(), 'G') + (double)count(tmp.begin(), tmp.end(), 'C')) / (double)n;
        if (GC_Ratio < temp) {
            ans = tmp;
            GC_Ratio = temp;
        }
    }
    cout << ans << endl;
    return 0;
}