77. Combinations java solutions

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 {
    List<List<Integer>> ans = new ArrayList<List<Integer>>();
    public List<List<Integer>> combine(int n, int k) {
        List<Integer> list = new ArrayList<>();
        combination(list,n,k,1);
        return ans;
    }
    
    public void combination(List<Integer> list, int n, int k,int s){
        if(k == 0){
            List<Integer> tmp = new ArrayList<>(list);
            ans.add(tmp);
            return;
        }
        for(int i = s; i <= n; i++){
            list.add(i);
            combination(list,n,k-1,i+1);
            list.remove(list.size()-1);
        }
    }
}