用ASCII码来解决

import java.util.*;
public class Main {
    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        int[] arr = new int [26];
        char c;
        for(int i = 0; i < s.length(); i ++)
        {
            c = s.charAt(i);
            arr[(int)c - 97]++;
        }
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] > 0) {
                System.out.print((char)(i+97));
                System.out.print(arr[i]);
            }
        }
    }    
}