leetcode Subsets II

Given a collection of integers that might contain duplicates, 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 = [1,2,2], a solution is:

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

normally we would just iterate from 0-2^n-1 and get the corresponding subset for each number. however, this one has duplicates. so the idea is different.

public class Solution {
public ArrayList<ArrayList<Integer>> subsetsWithDup(int[] num) {
    // Start typing your Java solution below
    // DO NOT write main() function
    Arrays.sort(num);
    ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
    ret.add(new ArrayList<Integer>());

    int start = 0;
    for(int i = 0; i < num.length; i++)
    {
        int size = ret.size();
        for(int j = start; j < size; j++)
        {
            ArrayList<Integer> sub = new ArrayList<Integer>(ret.get(j));
            sub.add(num[i]);
            ret.add(sub);
        }
        if(i < num.length - 1 && num[i + 1] == num[i])
            start = size;
        else
            start = 0;
    }

    return ret;
}
}