题解 | #MP3光标位置#

1:光标初始位置是1

2:歌曲数量大于4才会出现翻页的操作

3:翻页后光标还在第一个位置（向上翻）或第四个位置（向下翻）

while 1:
    try:
        n = int(input())
        action = input()
        cursor = 1
        screen = []
        for i in range(1,n+1):
            if i > 4:
                break #当歌曲数量大于4时，screen初始状态为[1,2,3,4]
            screen.append(i)#定义一个列表screen，当歌曲数量小于4，len(screen)=n, else: len(screen)=4
        for i in action:
            if i == 'U':
                cursor = cursor - 1
                if cursor == 0:#当向上翻到头时
                    cursor = n#光标指到最后一首歌
                    if n > 4:
                        screen = [n-3,n-2,n-1,n]#屏幕变成最后四首歌
                if cursor not in screen:
                    screen.insert(0,cursor)#屏幕随着光标往上翻页
                    screen.pop(4)
            if i == "D":
                cursor = cursor + 1
                if cursor > n:#当向下翻到头时
                    cursor = 1
                    if n > 4:
                        screen = [1,2,3,4]#屏幕变成最初四首歌
                if cursor not in screen:
                    screen.append(cursor)#屏幕随着光标向下翻页
                    screen.pop(0)
        print(" ".join(list(map(str,screen))))
        print(cursor)
    except:
        break