39. Combination Sum

def backtrackCombSum(self, nums, target, idx, temp, res):
    if target < 0:
        return
    if target == 0:
        res.append(temp)
        return
    for i in range(idx, len(nums)):
        self.backtrackCombSum(nums, target - nums[i], i, temp + [nums[i]], res)

    return


def combinationSum(self, candidates, target):
    """
    :type candidates: List[int]
    :type target: int
    :rtype: List[List[int]]
    """
    res = []
    self.backtrackCombSum(candidates, target, 0, [], res)
    return res