小红书笔试第一次字符串压缩求助

一直都之A 80%，实在想不到哪里错了。有大佬全A了么？

附上我的代码：
import java.util.Scanner;

public class RedBookMain1 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        if(s== null || s.length() == 0){
            System.out.println(s);
        }
        char pre = s.charAt(0);
        long cnt = 0;
        StringBuilder res = new StringBuilder("");
        for(int i = 1;i < s.length(); i++){
            char c = s.charAt(i);
            if(!Character.isAlphabetic(c)){
                res = res.append(c);
                continue;
            }
            if(c == pre){
                cnt++;
            }else{
                if(cnt == 0){
                    res = res.append(pre);
                }else{
                    res = res.append(cnt).append(pre);
                }
                cnt = 0;
                pre = c;
            }
        }
        if(cnt == 0){
            res = res.append(pre);
        }else{
            res = res.append(cnt).append(pre);
        }
        System.out.println(res.toString());
    }
}