题解 | #【模板】栈#

n = int(input())  
a = []  
for i in range(n):  
    b = input().split(" ")  
    if b[0] == "push":  
        a.append(int(b[1]))  
    elif b[0] == "pop":  
        if len(a) == 0:  
            print("error")  
        else:  
            c = a.pop(-1)  
            print(c)  
    else:  
        if len(a) == 0:  
            print("error")  
        else:  
            print(a[-1])

elif 和else的巧妙运用