题解 | 【模板】队列操作

from sys import stdin
from collections import *
from random import *
input = lambda: stdin.readline().strip()
# read1 = stdin.read().split()
# idx = 1
l = deque()
for _ in range(int(input())):
    x = input().split()
    if x[0] == "1":
        l.append(x[1])
    elif x[0] == "2":
        try:
            l.popleft()
        except:
            print("ERR_CANNOT_POP")
    elif x[0] == "3":
        try:
            print(l[0])
        except:
            print("ERR_CANNOT_QUERY")
    else:
        print(len(l))