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:

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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:

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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);         }     } }