[Leetcode] Subsets

Given a set of distinct integers, S, return all possible subsets.

Note:

Elements in a subset must be in non-descending order.
The solution set must not contain duplicate subsets.

For example,
If S = [1,2,3], a solution is:

[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

Solution:

 1 public class Subsets {
 2     public List<List<Integer>> subsets(int[] S) {
 3         List<List<Integer>> ret = new ArrayList<List<Integer>>();
 4         List<Integer> path = new ArrayList<Integer>();
 5         
 6         Arrays.sort(S);
 7         
 8         subsets(S, path, ret, 0);
 9         
10         return ret;
11     }
12     
13     public void subsets(int[] S, List<Integer> path, List<List<Integer>> ret, int index) {
14         // 把当前的结果可以添加到结果集中. 空集也算是一种集合 
15         ret.add(new ArrayList<Integer>(path));
16         
17         for (int i = index; i < S.length; i++) {
18             path.add(S[i]);
19             
20             // 注意！这里的index要填写i + 1，而不是index，开始老是会犯错。
21             subsets(S, path, ret, i + 1);
22             path.remove(path.size() - 1);
23         }
24     }
25 }