# 思路：兩個佇列，佇列1只負責壓棧，佇列2只負責出棧
class Two_queue(object):
    def __init__(self):
        self._oneli=[]
        self._twoli=[]
    def entrance(self,item):
        '''佇列模擬進棧'''
        self._oneli.append(item)
    def popup(self):
        '''佇列模擬出棧'''
        if len(self._oneli)==0:
            return None
        while len(self._oneli)!=1:
            # 將佇列1的元素彈出給佇列2，留下一個元素用來彈出
            # 因為最終彈出的是後進的，所以從頭部彈出
            self._twoli.append(self._oneli.pop(0))
        # 交換佇列1和佇列2的位置
        self._oneli,self._twoli=self._twoli,self._oneli
        return self._twoli.pop(0)
if __name__ == '__main__':
    stacks=Two_queue()
    stacks.entrance(1)
    stacks.entrance(2)
    stacks.entrance(3)
    stacks.entrance(4)
    stacks.entrance(5)
    print(stacks.popup())
    print(stacks.popup())
    print(stacks.popup())
    print(stacks.popup())
    print(stacks.popup())