vector<string> combinations(int n){
    vector<char> digits = {'0','1','2','3','4,'5','6','7','8','9'};

    string s;
    for ( int i = 0; i < n; i++ ){
        char ch = pick digits;
        s += ch;
    }
    return s;
}

====

vector<string> combinations(int n){
    vector<char> digits = {'0','1','2','3','4,'5','6','7','8','9'};

    string s;
    for ( int i = 0; i < n; i++ ){
		return pick(digits, [](char ch){
			s += ch;
			for ( ; i < n; i++ ){
				return pick(....
					);
			}
			return s;
		});
    }

}