import java.util.*; public class Solution { ArrayList<ArrayList<Integer>> res = new ArrayList<>(); ArrayList<Integer> track = new ArrayList<>(); public ArrayList<ArrayList<Integer>> combine (int n, int k) { backtrack(1,n,k); return res; } private void backtrack(int start,int n,int k){ if(track.size() == k){ res.add(new ArrayList(track)); return; } for(int i = start;i<=n;i++){ track.add(i); backtrack(i+1,n,k); track.remove(track.size()-1); } } }