Leetcode: 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],
  []
]

Understand the problem:
As described in the problem, given a set of DISTINCT integers, S, return all possible subsets. Elements in a subset must be in non-descending order. The solution set must not contain duplicate subsets. 


Recursive Solution:

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;
        
        Arrays.sort(S);
        
        result.add(new ArrayList<Integer>());
        
        ArrayList<Integer> temp = new ArrayList<Integer>();
        subsetsHelper(S, 0, result, temp);
        
        return result;
    }
    
    private void subsetsHelper(int[] S, int start, ArrayList<ArrayList<Integer>> result, ArrayList<Integer> temp) {
        if (start >= S.length) return;
        
        for (int i = start; i < S.length; i++) {
            temp.add(S[i]);
            result.add(new ArrayList<Integer>(temp));
            subsetsHelper(S, i + 1, result, temp);
            
            temp.remove(temp.size() - 1);
        }
    }
}