题解 | #【模板】栈#

import sys

class ListNode:

    def __init__(self, val) -> None:
        
        self.val = val

        self.next = None

class LinkedList:

    def __init__(self) -> None:
        
        self.head = None
    
    def pop(self):

        if self.head == None:

            print('error')

        else: 
             
            print(self.head.val)

            self.head = self.head.next
    
    def top(self):

        if self.head == None:

            print('error')
        
        else:

            print(self.head.val)

    def push(self,val):

        if self.head == None:

            self.head = ListNode(val)


        else:
             
            newNode = ListNode(val)

            newNode.next = self.head

            self.head = newNode

def is_space(string):
    
    for i in range(len(string)):

        if string[i] == ' ':

            return True

    return False

def main():

    input_data = sys.stdin.read()

    lines = input_data.strip().split('\n')

    time = int(lines[0])

    linkedList = LinkedList()

    for i in range(1,time+1):

        if is_space(lines[i]):

            com, data = lines[i].split(' ')

            linkedList.push(int(data))

        elif lines[i] == 'top':

            linkedList.top()
        
        elif lines[i] == 'pop':

            linkedList.pop()

if __name__ == '__main__':

    main()