题解 | #MP3光标位置#

import java.util.Scanner;
import java.util.Arrays;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNext()) { // 注意 while 处理多个 case
            int size = in.nextInt();
            String command = in.next();

            //返回两个结果，一个是光标位置，一个是当前页面展示的菜单
            int index = 0;
            int[] songs = new int[size];
            for (int i = 0; i < size; i++) {
                songs[i] = i + 1;
            }

            int startIndex = 0;
            int endIndex = Math.min(3, size - 1);
            for (int i = 0; i < command.length(); i++) {
                if (command.charAt(i) == 'D') {
                    index++;
                } else if (command.charAt(i) == 'U') {
                    index--;
                } else {
                    continue;
                }
                //如果下标在子数组范围内，则不变；否则，移动数组边界
                if (index >= startIndex && index <= endIndex) {
                    continue;
                }
                //如果超过的右边界，且没超过数组尾部，则向右移动数组边界；否则，向左移动数组边界
                if (index > endIndex && index < size - 1) {
                    endIndex++;
                    startIndex++;
                } else if (index < startIndex && index > 0) {
                    startIndex--;
                    endIndex--;
                }
                //超出数组边界，则从头或者从尾部开始
                if (index < 0) {
                    endIndex = size - 1;
                    startIndex = Math.max(0, endIndex - 3);
                    index = endIndex;
                }
                if (index > songs.length - 1) {
                    startIndex = 0;
                    endIndex = Math.min(3, size - 1);
                    index = startIndex;
                }
            }
            int[] currentSongs = Arrays.copyOfRange(songs, startIndex, endIndex + 1);
            for (int currentSong : currentSongs) {
                System.out.print(currentSong + " ");
            }
            System.out.println();
            //打印当前选中的歌曲
            System.out.println(songs[index]);
        }
    }
}