利用正则表达式解题，简单，粗暴，重要的是还很快

解题思路:不懂正则表达式的可以去CSDN搜索该博客，有详细资料，包你看完就秒懂正则表达式

注意事项:https://mp.csdn.net/mp_blog/creation/editor/122734157
参考代码:

import java.util.Scanner;

import java.util.regex.*;

public class Main {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

while(sc.hasNext()) {

String st = sc.nextLine();

if(st.equals("End of file")) {

break;

}

String regstr = "^(([0-9]|[1-9][0-9]|[1][0-9][0-9]|[2][0-5][0-5])\\.){3}([0-9]|[1-9][0-9]|[1][0-9][0-9]|[2][0-5][0-5])$";

    boolean f = Pattern.matches(regstr,st);

if(f) {

System.out.println("Y");

}else {

System.out.println("N");

}

}

        sc.close();

}

}