C++ Combinatorics Combinations high performance function - c++

Question

Here is my function for Combinatorics Combinations.
For example: combinations "ABCD", 2 = AB AC AD BC BD CD.
In future, i will do some operations with each combination(not just printf).
I wonder, is there a way to improve perfomance of this code?
#include "stdafx.h"
#include "iostream"
#include <vector>
void display(std::vector<char> v, int* indices, int r)//f() to display combinations
{
for (int i = 0; i < r; ++i)
std::cout << v[indices[i]];
std::cout << std::endl;
}
void combinations(std::vector<char> v, int n, int r, void(*f)(std::vector<char>, int*, int))
{
int* indices = new int[r];
for (int i = 0; i < r; ++i)
indices[i] = i;
int count;
bool b;
f(v, indices, r);
while (true)
{
b = true;
for (count = r - 1; count >= 0; --count)
{
if (indices[count] != count + n - r)
{
b = false;
break;
}
}
if (b)
break;
++indices[count];
for (int i = count + 1; i < r; ++i)
indices[i] = indices[i - 1] + 1;
f(v, indices, r);
}
delete[] indices;
}
int _tmain(int argc, _TCHAR* argv[])
{
std::vector<char> v(4);//pool
v[0] = 'A';
v[1] = 'B';
v[2] = 'C';
v[3] = 'D';
int n = 4;// pool size
int r = 2;// length of each combination
combinations(v, n, r, display);// pool, pool size, len of combination, func for each combination
return 0;
}

Answer 1

Maybe not performance but readability is also important.
See a solution in recursion.
http://cpp.sh/2jimb
#include <iostream>
#include <string>
typedef unsigned int uint;
typedef std::string::iterator seq_iterator;
std::string FindCombinations(seq_iterator current, seq_iterator end, uint comb_length, std::string comb)
{
if (comb_length == 0 || comb_length > std::distance(current, end))
return comb;//no more possible combinations
auto next = current + 1;
std::string with;
if (std::distance(next, end) >= comb_length - 1)
with = FindCombinations(next, end, comb_length - 1, comb + *current);//use current in combination
std::string without;
if (std::distance(next, end) >= comb_length)
without = FindCombinations(next, end, comb_length, comb);//skip current in combination
std::string result = with;
if (!result.empty() && !without.empty())
result += ' ';
return result + without;
}
int main()
{
std::string seq = "ABCDE";
std::cout << FindCombinations(seq.begin(), seq.end(), 2, "") << std::endl;
}