Given an array, print all possible combinations of it's elements. (Note that this would be the equivalent of calling an n-choose-k function n - 1 times where k is 1 on the first call and incremented by one on each subsequent call.)
Example:

Input: a b c
Output: a, a b, a b c, a c, b, b c, c

void Combinations(vector<char> &arr)
{
    vector<char> res;
    CombinationsHelper(arr, res, 0);
}
 
void CombinationsHelper(vector<char> &arr, vector<char> res, int index)
{
    if (index == arr.size())
        return;
 
    for (int i = index; i < arr.size(); i++)
    {
        res.push_back(arr[i]);
        
        for (char c : res)
            cout << c;
        cout << endl;
 
        CombinationsHelper(arr, res, i + 1);
 
        res.pop_back();
    }
}