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。

public class Solution {
    public ArrayList<ArrayList<Integer>> subsets(int[] S) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        for(int i = 0; i < Math.pow(2, S.length); i ++){
            ArrayList<Integer> row = new ArrayList<Integer>();
            for(int j = 0; j < S.length; j ++){
                if((i >> j & 1) == 1) row.add(S[j]);
            }
            result.add(row);
        }
        return result;
    }
}

这个做法 的顺序不对。 改进后如下：

public class Solution {
    public ArrayList<ArrayList<Integer>> subsets(int[] S) {
        // Start typing your Java solution below
        // DO NOT write main() function
        Arrays.sort(S);
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        for(int i = 0; i < (1 << S.length); i ++){
            ArrayList<Integer> row = new ArrayList<Integer>();
            for(int j = 0; j < S.length; j ++){
                if((i & (1 << j)) != 0) row.add(S[j]);
            }
            result.add(row);
        }
        return result;
    }
}