https://www.codeeval.com/open_challenges/38/

string_list.cpp

#include <iostream>
#include <sstream>
#include <fstream>
#include <stdlib.h>
#include <iterator>
#include <vector>
#include <set>

using namespace std;

void permutate(const unsigned int length,
               const string& word,
               vector<int>& index,
               size_t depth,
               set<string>& words)
{
    if (length == 1) {
        for (size_t i = 0; i < word.length(); ++i) {
            words.insert(string(1, word[i]));
        }
        return;
    }

    if (depth >= length) {
        string permutation;
        for (size_t i = 0; i < length; ++i) {
            permutation.push_back(word[index[i]]);
        }
        words.insert(permutation);
        return;
    }

    for (size_t i = 0; i < word.length(); ++i) {
        index[depth] = static_cast<int>(i);
        permutate(length, word, index, depth + 1, words);
    }
}

int main(int argc, char *argv[]) {
    if (argc < 2) return -1;

    ifstream datafile(argv[1]);
    string line;

    while (getline(datafile, line)) {
        if (line.length() < 1) continue;

        int separator_index = line.find_first_of(",");

        unsigned int length_as_int;
        size_t length = 0;
        string letters = "";

        istringstream length_part(line.substr(0, separator_index));
        length_part >> length_as_int;
        length = static_cast<size_t>(length_as_int);

        letters = line.substr(separator_index + 1, line.length());

        vector<int> index(length);

        ostringstream out;
        set<string> words;

        permutate(length, letters, index, 0, words);

        copy(
            words.begin(),
            words.end(),
            ostream_iterator<string>(out, ",")
        );

        string output = out.str();
        output.resize(output.length() - 1);
        cout << output << endl;
    }

    return 0;
}