LeetCode 61 Subsets II

Given a collection of integers that might contain duplicates, S, return all possible subsets.
Note:
1, Elements in a subset must be in non-descending order.
2, 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],
  []
]

分析:
return allを見てまずdfsが解決できるかどうかを考えます.
num配列を並べ替えると、同じ要素が隣接し、重複を判断できます.
配列を巡り、各要素をitemに加え、後ろの部分を再帰的に検索します.

public class Solution {
    public List<List<Integer>> subsetsWithDup(int[] num) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        List<Integer> item = new ArrayList<Integer>();
        Arrays.sort(num);
        result.add(item);//empty set
        dfs(result, item, num, 0);
        return result;
    }
    
    private void dfs(List<List<Integer>> result, List<Integer> item, int[] num, int pos){
        for(int i=pos; i<num.length; i++){
            item.add(num[i]);
            result.add(new ArrayList<Integer>(item));
            dfs(result, item, num, i+1);
            item.remove(item.size()-1);
            while(i<num.length-1 && num[i]==num[i+1])
                i++;//ignore duplicate
        }
    }
}