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],

  []

]
这题和Combinations一样。 也可以用位运算做。

 1 public class Solution {

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

 3         // Start typing your Java solution below

 4         // DO NOT write main() function

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

 6         for(int i = 0; i < Math.pow(2, S.length); i ++){

 7             ArrayList<Integer> row = new ArrayList<Integer>();

 8             for(int j = 0; j < S.length; j ++){

 9                 if((i >> j & 1) == 1) row.add(S[j]);

10             }

11             result.add(row);

12         }

13         return result;

14     }

15 }

这个做法 的顺序不对。 改进后如下：

 1 public class Solution {

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

 3         // Start typing your Java solution below

 4         // DO NOT write main() function

 5         Arrays.sort(S);

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

 7         for(int i = 0; i < (1 << S.length); i ++){

 8             ArrayList<Integer> row = new ArrayList<Integer>();

 9             for(int j = 0; j < S.length; j ++){

10                 if((i & (1 << j)) != 0) row.add(S[j]);

11             }

12             result.add(row);

13         }

14         return result;

15     }

16 }