[LeetCode] Combinations

Problem:

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],
]

Solution:O(n)

public class Solution {
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        ArrayList<ArrayList<Integer>> list = new ArrayList<>();
        if(k > n || k <1)
            return list;
        
        if(k == 1){
            for(int i=1; i<=n; i++){
                ArrayList<Integer> slist = new ArrayList<>();
                slist.add(i);
                list.add(slist);
            }
        }else{
            ArrayList<ArrayList<Integer>> slist1 = combine(n-1, k-1);
            ArrayList<ArrayList<Integer>> slist2 = combine(n-1, k);
            for(int i=0; i<slist1.size();i++){
                slist1.get(i).add(n);
                list.add(slist1.get(i));
            }
            for(int i=0; i<slist2.size();i++){
                list.add(slist2.get(i));
            }
        }    
        return list;
    }
}