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],
]

public class Solution {
    /**This code is an implemenatation of bfs idea.<br>
     * @author Averill Zheng
     * @version 2014-06-05
     * @since JDK 1.7
     */ 
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer> > combinations = new ArrayList<List<Integer> >();
		if(k > 0){
			for(int i = 0; i < n; ++i){
				List<Integer> aList = new ArrayList<Integer>();
				aList.add(i + 1);
				combinations.add(aList);
			}
			if(k > 1){
				for(int j = 1; j < k; ++j){ //j denotes the number of integer in each list
					List<List<Integer>> temp = new ArrayList<List<Integer> >();
					int length = combinations.size();
					for(int l = 0; l < length; ++l){
						List<Integer> aList = combinations.get(l);
						int lastNumber = aList.get(j - 1);
						if(n - lastNumber >= k - j){ // has enough number to get the k combinations
							for(int m = lastNumber + 1; m <= n; ++m){
								List<Integer> newList = new ArrayList<Integer>();
								newList.addAll(aList);
								newList.add(m);
								temp.add(newList);
							}
						}
					}
					combinations = temp;
				}
			}
		}	
        return combinations;    
    }
}