Given a collection of integers that might contain duplicates, nums, return all possible subsets.

Note: The solution set must not contain duplicate subsets.

For example,
If nums = [1,2,2], a solution is:

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

一刷：
DFS + backtracking

public class Solution {
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if(nums == null || nums.length == 0) return res;
        List<Integer> list = new ArrayList<>();
        Arrays.sort(nums);
        dfs(nums, res, list, 0);
        return res;
    }
    
    private void dfs(int[] nums, List<List<Integer>> res, List<Integer> list, int pos){
        res.add(new ArrayList<>(list));
        
        for(int i=pos; i<nums.length; i++){
            if(i>pos && nums[i] == nums[i-1]) continue;
            list.add(nums[i]);
            dfs(nums, res, list, i+1);
            list.remove(list.size()-1);
        }
    }
}