[LeetCode] Combinations

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

#include <iostream>
#include <vector>
using namespace std;

vector< vector<int> > ret;
vector<int> a;

void build(int dep, int maxDep, int n, int start)
{
    if (dep == maxDep)
    {
        ret.push_back(a);
        return;
    }

    for (int i = start; i <= n; ++i)
    {
        a[dep] = i;
        build(dep + 1, maxDep, n, i + 1);
    }
}

vector< vector<int> > combine(int n, int k)
{
    a.resize(k);
    ret.clear();
    build(0, k, n, 1);
    return ret;
}

int main()
{
    vector< vector<int> > vec = combine(4, 2);
    for (int i = 0; i < vec.size(); ++i)
    {
        for (int j = 0; j < vec[0].size(); ++j)
        {
            cout << vec[i][j] << " ";
        }
        cout << endl;
    }
}