[leetCode] Combination Sum

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

The same repeated number may be chosen from C unlimited number of times.

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 2,3,6,7 and target 7, 
A solution set is: 
[7] 
[2, 2, 3] 

public class Solution {    List<List<Integer>> res;    public List<List<Integer>> combinationSum(int[] candidates, int target) {        res = new ArrayList<List<Integer>>();        Arrays.sort(candidates);        for (int i = 0; i < candidates.length; i++) {            List<Integer> list = new ArrayList<Integer>();            helper(candidates, list, i, 0, target);        }        return res;    }    public void helper(int[] candidates, List<Integer> list, int pos, int sum, int target) {        sum += candidates[pos];        if (sum > target) return;        else if (sum == target) {            list.add(candidates[pos]);            List<Integer> tmp = new ArrayList<Integer>(list);            res.add(tmp);            list.remove(list.size()-1);            return;        }        else {            list.add(candidates[pos]);            for (int i = pos; i < candidates.length; i++) {                helper(candidates, list, i, sum, target);            }            list.remove(list.size()-1);        }    }}