Given a set of characters represented by a String, return a list containing all subsets of the characters.

Assumptions:

There are no duplicate characters in the original set.

​Examples

Set = "abc", all the subsets are [“”, “a”, “ab”, “abc”, “ac”, “b”, “bc”, “c”]

Set = "", all the subsets are [""]

Set = null, all the subsets are []

Approach: DFS, with n levels, each level whether add the character or not.

public class Solution{
    public List<List<characters>> subsets(String ) {
    }
}