用python的list模擬stack
n = int(input())

class stack:
    def __init__(self) -> None:
        self.items = []
    def push(self,x):
        self.items.append(x)
    def pop(self):
        if self.items:
            print(self.items.pop())
        else:
            print("error")
    def top(self):
        if self.items:
            print(self.items[-1])
        else:
            print("error")

stack1 = stack()
for _ in range(n):
    a = input().strip().split()
    if len(a) == 1:
        if a[0] == "pop":
            stack1.pop()
        else:
            stack1.top()
    else:
        stack1.push(a[1])