All Subsets I

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 []

time: O(2 ^ n), space: O(2 ^ n)

public class Solution {
  public List<String> subSets(String set) {
    // Write your solution here.
    List<String> res = new ArrayList<>();
    if(set == null) {
      return res;
    }
    dfs(set, 0, new StringBuilder(), res);
    return res;
  }
  
  private void dfs(String set, int idx, StringBuilder sb, List<String> res) {
    if(idx == set.length()) {
      res.add(sb.toString());
      return;
    }
    
    sb.append(set.charAt(idx));
    dfs(set, idx + 1, sb, res);
    sb.deleteCharAt(sb.length() - 1);
    
    dfs(set, idx + 1, sb, res);
  }
}