Leetcode: Combination Sum

Question

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]

Solution

class Solution(object):
    def combinationSum(self, candidates, target):
        """ :type candidates: List[int] :type target: int :rtype: List[List[int]] """

        res = []
        if candidates==None or candidates==[]:
            return res

        candidates.sort()
        self.helper(candidates, 0, target, [], res)

        return res

    def helper(self, candidates, start, target, item, res):
        if target<0:
            return 
        if target==0:
            res.append(item)
            return

        for ind in range(start,len(candidates)):
            if ind>0 and candidates[ind]==candidates[ind-1]:
                continue
            self.helper(candidates, ind, target-candidates[ind], item+[candidates[ind]], res)