Combinations

1. Title

Combinations

2. Http address

https://leetcode.com/problems/combinations/

3. The question

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],
]

4. My code(AC)

// Accepted
       public List<List<Integer>> combine(int n, int k) {
           List<List<Integer>>  result = new ArrayList<List<Integer>>();
           List<Integer> subList = null;
           for(int begin = 1;  begin <= n; begin++)
           {
               subList = new ArrayList<Integer>();
               subList.add(begin);
               getCombinations(begin + 1, n, k, subList, result);
           }
          return result;
        }
       
       public void getCombinations(int begin, int end, int k, List<Integer> subResult, List<List<Integer>> result)
       {
           
           if( subResult.size() == k)
           {
                result.add(new ArrayList<Integer>(subResult));
               return;
           }
           for(int i = begin ; i <= end ; i++)
           {
               subResult.add(i);
               getCombinations(i + 1, end, k , subResult, result);
               subResult.remove(subResult.size() - 1);
           }
       }