Combination Sum II

Given a collection of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

Each number in C may only be used once in the combination.

Note:

• All numbers (including target) will be positive integers.
• Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
• The solution set must not contain duplicate combinations.

For example, given candidate set 10,1,2,7,6,1,5 and target 8,
A solution set is:
[1, 7]
[1, 2, 5]
[2, 6]
[1, 1, 6] 

Solution:
Typical dfs question. 
For case {1,1,1,6} 8, there are duplicates {1st,1nd,6} and {1nd,1rd,6}, so use Hash Set to clean it. 

public class Solution {
    public ArrayList> combinationSum2(int[] num, int target) {
        Arrays.sort(num);
        ArrayList> result = new ArrayList>();
        ArrayList subset = new ArrayList();
        helper(num, target, 0, result, subset);
        
        // get rid of duplicates
        HashSet> set = new HashSet>();
        set.addAll(result);
        result.clear();
        result.addAll(set);
        
        return result;
    }
    public void helper(int[] num, int target, int begin, ArrayList> result, ArrayList subset){
        if(target == 0){
            result.add(new ArrayList(subset));
            return;
        }
        
        for(int i = begin; i < num.length; i++){
            if(target >= num[i]){
                subset.add(num[i]);
                helper(num, target-num[i], i+1, result, subset);
                subset.remove(subset.size()-1);
            }
        }
    }
}