def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def combinationSum(self, candidates, target):
        candidates.sort()
        self.result = []
        self.dfs(candidates,target,0,[])
        return self.result