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],]

class Solution {
public:
    void DFS(int n,int k,vector<vector<int>> &result,vector<int> &path,int start)
    {
        if(path.size()==k)
          {
              result.push_back(path);
              return;
          }
        for(int i=start;i<=n;i++)  //从1开始算的，所有i到n
        {
            path.push_back(i);
            DFS(n,k,result,path,i+1);
            path.pop_back();
        }
    }
    
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> result;
        vector<int> path;
        DFS(n,k,result,path,1);
        return result;
    }
};