small combine function

combine.cpp

#include <vector>
#include <string>

template <typename T>
void combineInner(const std::vector<T> &data, int start, int m, int depth, std::vector<T> &temp, std::vector<std::vector<T>> &result)
{
    if (depth > m - 1)
        return;

    if (depth == m - 1)
    {
        for (std::size_t i = start; i < data.size(); ++i)
        {
            temp[depth] = data[i];
            result.push_back(temp);
        }
    }
    else
    {
        for (std::size_t i = start; i < data.size(); ++i)
        {
            temp[depth] = data[i];
            combineInner(data, i + 1, m, depth + 1, temp, result);
        }
    }
}

// choose m item from data, put all the result to result_list
template <typename T>
void combine(const std::vector<T> &data, std::size_t m, std::vector<std::vector<T>> &result_list)
{
    result_list.clear();
    if (m <= 0 || data.size() < m)
        return;

    if (data.size() == m)
    {
        result_list.push_back(data);
        return;
    }

    int depth = 0;
    std::vector<T> temp(m, T());
    combineInner(data, 0, m, depth, temp, result_list);
}

// g++ -g -std=c++11 -Wall -Wextra -o a.out combine.cpp
int main()
{
    {
        std::vector<int> a = {1, 2, 1, 4};
        std::vector<std::vector<int>> result;
        combine(a, 0, result);
        assert(result.size() == 0);
    }
    {
        std::vector<int> a = {1, 2, 1, 4};
        std::vector<std::vector<int>> result;
        combine(a, 1, result);
        assert(result.size() == 4);
    }
    {
        std::vector<int> a = {1, 2, 1, 4};
        std::vector<std::vector<int>> result;
        combine(a, 2, result);
        assert(result.size() == 6);
    }

    {
        std::string s("ABCDEF");
        std::vector<char> vchar(s.begin(), s.end());
        std::vector<std::vector<char>> result;
        combine(vchar, 2, result);
        assert(result.size() == 15);
        for (std::vector<char> item : result)
        {
            std::string s1(item.begin(), item.end());
            // std::cout << s1 << std::endl;
        }
    }
    return 0;
}