堆栈的使用

解题思路:

注意事项:

参考代码:

def stack_operation(commands):

    stack = []

    output = []

    for command in commands:

        if command[0] == 'P':

            stack.append(int(command[2:]))

        elif command[0] == 'O':

            if stack:

                stack.pop()

        elif command[0] == 'A':

            if stack:

                output.append(str(stack[-1]))

            else:

                output.append('E')

    return output

while True:

    n = int(input())

    if n == 0:

        break

    commands = []

    for _ in range(n):

        commands.append(input())

    result = stack_operation(commands)

    print('\n'.join(result))

    print()