[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+;
             DFS(Count+, i+);
         }
     }
     vector<vector<int> > combine(int n, int k) {
         ans.clear();
         vi.resize(k);
         k_start = k;
         n_start = n;
         DFS(,);
         return ans;
     }
 };