In this case, ensuring that pop() throws an exception if push() hasn't been called ( or logically, if the stack is empty) means that the stack has a state. 在这种情况下,如果push()没有被调用(或者从逻辑上讲,栈为空)的情况下确保pop()抛出一个异常,则意味着栈有一个状态。
Initialize an empty stack and empty result string variable. 初始化一个空堆栈,将结果字符串变量置空。
The equivalence of acceptance words by PDLA with final state and with empty stack is proved. 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。