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],
]

C ode:

 public class Solution {
     public ArrayList> combine(int n, int k) {
         ArrayList list = new ArrayList();
         ArrayList> listsAll = new ArrayList>();
         allCombine(n, k, 1, 0, list, listsAll);
         return listsAll;

     }

     public void allCombine(int n, int k, int current, int length, ArrayList list, ArrayList> listsAll) {
         if (current > n) return;
         list.add(current);
         length++;
         if (list.size() == k) {
             print(list,listsAll);
         }
         allCombine(n, k, current + 1, length, list, listsAll);
         list.remove(list.size() - 1);
         allCombine(n, k, current + 1, length, list, listsAll);
     }

     public void print(ArrayList list, ArrayList> listsAll) {
         ArrayList temp = new ArrayList(list);
         listsAll.add(temp);
     }
 }