Combinations [leetcode]

描述： Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

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

思路：模拟手工组合C(n,k)的方法， 用pos 标记当前的位置，从pos ，n 中间选取k个数据，

class Solution {
public:

    void choose(int n, int k, int pos, vector<int>&array, vector<vector<int> >&ret)
    {
        if(k == 0)
        {
            ret.push_back(array);
            return;
        }

        for(int i = pos ; i <= n; ++i)
        {
            array.push_back(i);
            choose(n, k-1, i+1, array, ret);
            array.resize(array.size()-1);
        }

        return;
    }

    vector<vector<int> > combine(int n, int k) 
    {
        vector<vector<int> > ret ;
        if(n == 0 || k == 0 || k > n)
            return ret;

        vector<int> array;
        choose(n, k, 1, array, ret);
        return ret;
    }
};