K digits optimal removal

void optimalDigitsRemoval(const string& s, int K) {
    vector < int > freq(10);
    int N = s.size(), stop = N;
    for(int i = 0; i < N; ++i) {
        int cif = s[i] - '0';
        for(int j = 0; j < cif; ++j)
            if(K >= freq[j]) {
                K -= freq[j];
                freq[j] = 0;
            }
            else {
                freq[j] -= K;
                K = 0;
            }
        if(K == 0) {
            stop = i;
            break;
        }
        ++freq[cif];
    }
    if(K > 0) {
        for(int i = 0; i < 10 && K > 0; ++i)
            if(K >= freq[i]) {
                K -= freq[i];
                freq[i] = 0;
            }
            else {
                freq[i] -= K;
                K = 0;
            }
    }
    for(int i = 9; i >= 0; --i)
        for(int j = 0; j < freq[i]; ++j)
            cout << i;
    for(int i = stop; i < N; ++i)
        cout << s[i];
}