90. Subsets II

Given a collection of integers that might contain duplicates, nums, return all possible subsets (the power set).

Note: The solution set must not contain duplicate subsets.

class Solution {
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        // sort array to prevent duplicatin in different recursion layers
        Arrays.sort(nums);
        dfs(nums, res, new ArrayList<>(), 0);
        return res;
    }
    private void dfs(int[] nums, List<List<Integer>> res, List<Integer> list, int start){
        res.add(new ArrayList<>(list));
        for(int i=start; i<nums.length; i++){
            // prevent duplication in the same recursion layer
            // hashset can also be used
            if (i>start && nums[i]==nums[i-1]) continue;
            list.add(nums[i]);
            dfs(nums, res, list, i+1);
            list.remove(list.size()-1);
        }
    }
}