class Browser:
    def _init_(self):
        self.__forward = []
        self.__current_url = []
        self.__backward = []

    def new_url(self,new_url):
        if len(self.__current_url) == 0:
            self.__current_url.append(new_url)
        else:
            self._backward.append(self._current_url[-1])
            self.__current_url.pop()
            self.__current_url.append(new_url)
        return self.__current_url[-1]

    def backwards(self):
        if len(self.__backward)  == 0:
            return None
        else:
            self._forward.append(self._current_url[-1])
            self._current_url.append(self._backward[-1])
            self.__backward.pop()
            self.__current_url.pop(0)
        return self.__current_url[-1]

    def  forward(self):
        if len(self.__forward)  == 0:
            return None
        else:
            self._backward.append(self._current_url[-1])
            self._current_url.append(self._forward[-1])
            self.__forward.pop()
            self.__current_url.pop(0)
        return self.__current_url[-1]