题解 | #【模板】栈#

class Stack:
    def __init__(self) -> None:
        self.items = []

    def push(self,item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

    def top(self):
        return self.items[len(self.items)-1]

if __name__ == '__main__':
    n = int(input())
    stack = Stack()
    for i in range(n):
        input_str = input().split()
        if input_str[0] == 'push':
            stack.push(int(input_str[1]))
        if input_str[0] == 'pop':
            if not stack.items:
                print('error')
            else:
                print(stack.pop())
        if input_str[0] == 'top':
            if not stack.items:
                print('error')
            else:
                print(stack.top())