Subsets II

public class Solution {
    public ArrayList<ArrayList<Integer>> subsetsWithDup(int[] num) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        res.add(new ArrayList<Integer>());
        if(num==null || num.length==0) return res;
        Arrays.sort(num);
        int start=0; 
        for(int i=0; i<num.length;i++){
            int len = res.size();
            for(int j= start; j<len; j++){
                ArrayList<Integer> tmp = new ArrayList<Integer>(res.get(j));
                tmp.add(num[i]);
                res.add(tmp);
            }
            if(i<num.length-1 && num[i]==num[i+1]){
                start = len; // let those processed data go for the dup one
            }else
                start =0;
        }
        return res;
    }
}