题解 | #合法IP#

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.lang.Integer;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String ip = br.readLine();
        String [] arr = ip.split("\\.");
        if(arr.length != 4){
            System.out.println("NO");
        }else{
            String res = "YES";
            for(String a:arr){
                if(a==null || a.length() == 0 ){
                    res = "NO";
                    break;
                }
                if(a.length() == 1){
                    if(!a.matches("[0-9]")){
                        res = "NO";
                        break;
                    }
                }else if(a.length() == 2){
                    if(!a.matches("[1-9]{1}[0-9]{1}")){
                        res = "NO";
                        break;
                    }
                }else if(a.length() == 3){
                    if(!a.matches("[1-9]{1}[0-9]{2}")){
                        res = "NO";
                        break;
                    }

                }else{
                    res = "NO";
                    break;
                }
                int n = Integer.parseInt(a);
                if(n < 0 || n > 255){
                    res = "NO";
                    break;
                }
            }
            System.out.println(res);
        }

    }
}