77. 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> combine(int n, int k) { List> res = new ArrayList>(); List al = new ArrayList(); combine(n, 1, k, res, al); return res; } public void combine(int n, int start, int k, List> res, List al) { if(al.size()==k) { res.add(new ArrayList(al)); return; } for(int i=start; i<=n; i++) { al.add(i); combine(n, i+1, k, res, al); al.remove(al.size()-1); } } }