Combinations of string - k at a time

Implement combinations of a string on length N, with K letters chosen at a time (N>K).  Eg:  The combination of string "abc" with 2 letters chosen at a time is - ab, ac, bc
/*
 * StringCombinationWithSize.cpp
 *
 *  Created on: Apr 18, 2012
 *      Author: Kiran Hegde
 */

#include <iostream>
#include <string>
#include <vector>

using namespace std;

void combine(string prefix, string rest, unsigned int k, vector<string> &output) {
    if (rest.size() < k)
        return;
    if (k == 0)
        output.push_back(prefix);
    for (unsigned int  i=0; i<rest.size(); i++) {
        combine(prefix + rest[i], rest.substr(i+1), k-1, output);
    }
}

/**
 * Test the function
 */
int main() {
    string input("abc");
    vector<string> combs;
    combine("", input, 2, combs); // choose 2 at a time
    cout << "Combinations: " << combs.size() << endl;
    for (unsigned int i=0; i<combs.size(); i++) {
        cout << combs[i] << endl;
    }
}
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