题解 | 【模板】队列操作

n = int(input())
m = []
for i in range(n):
    a = input().split()
    if a[0] == "1":
        m.append(a[1])
    elif a[0] == "2":
        if m:
            m.pop(0)
        else:
            print("ERR_CANNOT_POP")
    elif a[0] == "3":
        if m:
            print(m[0])
        else:
            print("ERR_CANNOT_QUERY")
    elif a[0] == "4":
        print(len(m))