[LeetCode 题解]：Combinations

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],
]
DFS，递归

class Solution {
public:
    vector<vector<int> > ans;
    vector<int> vi;
    int k_start,n_start;
    void DFS(int Count,int start)
    {
        if(Count== k_start)
        {
            ans.push_back(vi);
            return;
        }
        for(int i=start;i<n_start;i++)
        {
            vi[Count] = i+1;
            DFS(Count+1, i+1);
        }
    }
    vector<vector<int> > combine(int n, int k) {
        ans.clear();
        vi.resize(k);
        k_start = k;
        n_start = n;
        DFS(0,0);
        return ans;
    }
};