LeetCode-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],
]

class Solution {
public:
    void sub(vector<vector<int> >& ret,int n,int k){
        if(n<k)return;
        if(k==0)return;
        vector<vector<int> >copy=ret;
        for(int i=0;i<ret.size();i++){
            ret[i].push_back(n);
        }
        sub(ret,n-1,k-1);
        sub(copy,n-1,k);
        for(int i=0;i<copy.size();i++){
            ret.push_back(copy[i]);
        }
    }
    vector<vector<int> > combine(int n, int k) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<vector<int> >ret;
        ret.push_back(vector<int>());
        sub(ret,n,k);
        vector<vector<int> >ret2;
        for(int i=0;i<ret.size();i++){
            if(ret[i].size()==k){
                for(int j=0;j<k/2;j++){
                    int temp;
                    temp=ret[i][j];
                    ret[i][j]=ret[i][k-j-1];
                    ret[i][k-j-1]=temp;
                }
                ret2.push_back(ret[i]);
            }
        }

        return ret2;
    }
};