题解 | 【模板】队列操作

from collections import deque

# 初始化
queue = deque()

q = int(input())

for _ in range(q):
    operate = input().split()

    if operate[0] == '1':
        x = int(operate[1])
        queue.append(x)

    if operate[0] == '2':
        if queue:
            queue.popleft()
        else:
            print('ERR_CANNOT_POP')
    
    if operate[0] == '3':
        if queue:
            print(queue[0])
        else:
            print('ERR_CANNOT_QUERY')

    if operate[0] == '4':
        print(len(queue))