Leetcode: 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],
]

Understand the problem:
A classic permutation and combination problem. So using the recursion is a natural way. 

Solution:

public class Solution {
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if (n <= 0 || k <= 0 || n < k) {
            return result;
        }
        
        int[] num = new int[n];
        for (int i = 0; i < n; i++) {
            num[i] = i + 1;
        }
        ArrayList<Integer> curr = new ArrayList<Integer>();
        combineHelper(num, 0, 0, k, result, curr);
        
        return result;
    }
    
    private void combineHelper(int[] num, int start, int m, int k, 
        ArrayList<ArrayList<Integer>> result, ArrayList<Integer> curr) {

        if (m == k) {
            result.add(new ArrayList<Integer>(curr));
            return;
        }
        

        for (int i = start; i < num.length; i++) {
            curr.add(num[i]);
            combineHelper(num, i + 1, m + 1, k, result, curr);
            curr.remove(curr.size() - 1);
        }
    }
}

Update on 9/25/15:

public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (n <= 0 || k <= 0 || n < k) {
            return result;
        }
        
        List<Integer> curr = new ArrayList<>();
        
        combineHelper(1, n, 0, k, curr, result);
        
        return result;
    }
    
    private void combineHelper(int start, int n, int num, int k, 
       List<Integer> curr, List<List<Integer>> result) {
        if (num == k) {
            result.add(new ArrayList<Integer>(curr));
            return;
        }
        
        for (int i = start; i <= n; i++) {
            curr.add(i);
            combineHelper(i + 1, n, num + 1, k, curr, result);
            curr.remove(curr.size() - 1);
        }
    }
}