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],]

void comb(int n, int index, int k, vector<vector<int>> &res, vector<int> &path)    {    if(path.size() == k){res.push_back(path);return;}for(int i = index+1; i <=n; i++){path.push_back(i);comb(n,i,k,res,path);path.pop_back();}}vector<vector<int> > combine(int n, int k) {        // Note: The Solution object is instantiated only once        vector<vector<int>> res;if(n<k || n<1 || k<1)return res;vector<int> path;for(int i =1; i <= n; i++){path.push_back(i);comb(n,i,k,res,path);path.pop_back();}    }