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.
• 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]]

Difficulty: Medium

Solution: BackTracking

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