78. Subsets

题目：

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],

  []

]

链接： http://leetcode.com/problems/subsets/

题解：

求数组子数组。先把数组排序，之后就可以使用DFS，维护一个递增的position，递归后要backtracking。

Time Complexity - O(n * 2ⁿ)， Space Complexity - O(n)

public class Solution {

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

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

        if(S == null || S.length == 0)

            return result;

        ArrayList<Integer> list = new ArrayList<Integer>();

        Arrays.sort(S);

        helper(result, list, S, 0);

        return result;

    }

    

    private void helper(ArrayList<ArrayList<Integer>> result, ArrayList<Integer> list, int[] S, int pos){

        result.add(new ArrayList<Integer>(list));

        

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

            list.add(S[i]);

            helper(result, list, S, ++pos);

            list.remove(list.size() - 1);

        }

    }

}

 

测试：

Reference: 

http://www.cnblogs.com/springfor/p/3879830.html

http://www.cnblogs.com/zhuli19901106/p/3492515.html

http://www.1point3acres.com/bbs/thread-117602-1-1.html