Subset

Given a set of distinct integers, return all possible subsets.

Example

If S = [1,2,3], a solution is:

[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

Note

Elements in a subset must be in non-descending order.

For Question I: The solution set must not contain duplicate subsets. For Question II: The solution set may contain duplicate subsets.

Solution

// Question I：
public class Solution {

    /**
    * Recursive method, backtracking with a boolean array
    **/
    public ArrayList<ArrayList<Integer>> subsets(int[] num) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(num == null || num.length == 0) 
            return result;
        ArrayList<Integer> list = new ArrayList<Integer>();
        Arrays.sort(num);  
        subsetsHelper(result, list, num, 0);
        return result;
    }

    private void subsetsHelper(ArrayList<ArrayList<Integer>> result, ArrayList<Integer> list, int[] num, int pos) {
        result.add(new ArrayList<Integer>(list));
        for (int i = pos; i < num.length; i++) {
            list.add(num[i]);
            subsetsHelper(result, list, num, i + 1);
            list.remove(list.size() - 1);
        }
    }

    /**
    * Iterate method, a loop and retreve the value in original list and generate new value with insert it
    **/
    public ArrayList<ArrayList<Integer>> subsets(int[] num) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if(num == null || num.length == 0)
            return res;
        ArrayList<Integer> list = new ArrayList<Integer>();
        Arrays.sort(num);  
        res.add(list);
        for(int i = 0; i < num.length; i++) {
            int size = res.size();
            for(int j = 0; j < size; j++) {
                list = new ArrayList<>(res.get(j));
                list.add(num[i]);
                res.add(list);
            }
        }
        return res;
    }
}


// Question II:
public class Solution {
    public ArrayList<ArrayList<Integer>> subsetsWithDup(ArrayList<Integer> num) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(num == null || num.size() == 0) {
            return result;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        Collections.sort(num);  
        subsetsHelper(result, list, num, 0);

        return result;
    }


    private void subsetsHelper(ArrayList<ArrayList<Integer>> result,
        ArrayList<Integer> list, ArrayList<Integer> num, int pos) {

        result.add(new ArrayList<Integer>(list));

        for (int i = pos; i < num.size(); i++) {
            if(i > pos && num.get(i) == num.get(i-1))
                continue;
            list.add(num.get(i));
            subsetsHelper(result, list, num, i + 1);
            list.remove(list.size() - 1);
        }
    }
}