【leetcode刷题笔记】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],

  []

]

---

题解：实现一个递归的函数生成长度为level的所有子集放入结果中，然后循环的置这个level为0~S.length，即可生成所有的子集。

代码如下：

 1 public class Solution {

 2     public void subsetsDfs(int[] s,int level,int pos,ArrayList<Integer> result,List<List<Integer>> answer){

 3         if(result.size() == level){

 4             answer.add(new ArrayList<Integer>(result));

 5             return;

 6         }

 7         for(int i = pos;i < s.length;i++){

 8             result.add(s[i]);

 9             subsetsDfs(s, level, i+1, result, answer);

10             result.remove(result.size()-1);

11         }

12     }

13     public List<List<Integer>> subsets(int[] S) {

14         Arrays.sort(S);

15         List<List<Integer>> answer = new ArrayList<List<Integer>>();

16         ArrayList<Integer> result = new ArrayList<Integer>();

17         

18         for(int i = 0;i <= S.length;i++)

19             subsetsDfs(S, i, 0, result, answer);

20         return answer;

21     }

22 }