《空栈》是观风下棋创作的网络小说，发表于起点网。
下推格值自动机以终态与以空栈两种接受词语言的方式是等价的。
The equivalence of acceptance words by PDLA with final state and with empty stack is proved.
现在我可以为pop方法编写行为，即确保当栈在逻辑上为空时，抛出一个异常。
Now I can code the behavior for the pop method, which ensures that if the stack is logically empty, an exception will be thrown.
如果再次调用pop，那么应该返回foo，然后栈为空(假设在添加这两个对象之前栈中没有对象)。
If you called pop again, then foo should be returned and the stack should be empty (assuming there wasn't anything in it before you added the two objects).
应用推荐