permutation.cc

void permutation(string &s) {
    permu_helper(s, 0);
}


void permu_helper(string &s, int level) {
    if (level == s.size()) {
        cout << s << endl;
        return;
    }
    for (int i = level; i < s.size(); ++i) {
        swap(s[level], s[i]);
        permu_helper(s, level + 1);
        swap(s[level], s[i]);
    }
}