def connect(self): 
        try:
            self.connection = pymongo.Connection(self.host, self.port)
            self.db = self.connection[self.name]
        except AutoReconnect:
            raise DatabaseException("It's not possible connect to the database")
def combinationSum(self, candidates, target):
        self.result = []
        self.dfs(candidates,0,target,[])
        return self.result