可参考：https://www.cnblogs.com/panweiwei/p/14025143.html

class Solution(object):
    def __init__(self):
        self.res = []

    def combinationSum(self, candidates, target):
        """
        :type candidates: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        # 特判
        if not candidates:
            return []
        # 可以重复调用，所以先排个序，确保相同数字相邻
        candidates.sort()
        # 获取元素个数
        n = len(candidates)
        # 调用函数
        self.dfs(candidates, target, n, 0, 0, [])
        return self.res

    def dfs(self, candidates, target, n, begin, temp_sum, temp):
        if temp_sum > target or begin == n:
            return
        if temp_sum == target:
            self.res.append(temp)
            return
        for i in range(begin, n):
            if temp_sum + candidates[i] > target:
                break
            self.dfs(candidates, target, n, i, temp_sum + candidates[i], temp + [candidates[i]])