题解 | #合法IP#

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNext()) { 
            String s = in.next();
            boolean flag = false;
            //去掉头尾的.
            while(s.charAt(0) == '.'){
                s = s.substring(1,s.length());
            }
            while(s.charAt(s.length()-1) == '.'){
                s = s.substring(0,s.length()-1);
            }
            //两个.连续出现的情况
            for(int i = 0; i<s.length(); i++){
                if(s.charAt(i) == '.' && s.charAt(i+1) == '.'){
                    flag = true;
                    System.out.print("NO");
                    return;
                }
            }
            String[] ss = s.split("\\.");
            //判断每一部分是否合法
            outer:
            for(int i = 0; i<ss.length; i++){
                if(ss.length != 4 || (ss[i].charAt(0) == '0' && ss[i].length() != 1)){
                    flag = true;
                    break;
                }
                for(int j = 0; j<ss[i].length(); j++){
                    if(!Character.isDigit(ss[i].charAt(j)) || ss[i].length()>3 || Integer.valueOf(ss[i])<0 || Integer.valueOf(ss[i])>255){
                        flag = true;
                        break outer;
                    }
                }
            }
            System.out.print(flag?"NO":"YES");
        }
    }
}