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],
]

public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> res=new LinkedList< List<Integer>>();
        combine(res,n,1,k,new LinkedList<Integer>());
        return res;
    }
    public void combine(List<List<Integer>> res,int n,int start,int k,LinkedList<Integer> a){
        if(k==0){
            res.add(a);
            return;
        }
        for(int i=start;i<=n;i++){
            LinkedList<Integer> list=(LinkedList<Integer>)a.clone();
            list.add(i);
            combine(res,n,i+1,k-1,list);
        }
    }
}