Задача простая, но переделываю уже второй раз...Валидатор логает...
package com.javarush.task.task18.task1803;
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.*;
public class Solution {
public static void main(String[] args) throws Exception {
BufferedReader reader = new BufferedReader((new InputStreamReader(System.in)));
String filename = reader.readLine();
FileInputStream inputStream = new FileInputStream(filename);
List<Integer> list = new ArrayList<>();
List<Integer> list2 = new ArrayList<>();
while (inputStream.available() > 0)
{
int data = inputStream.read();
list.add(data);
}
Collections.sort(list);
int coin = 0;
int coin1 = 0;
for (int i = 0; i <list.size() ; i++) {
if (list.get(i)== list.get(i+1)){
coin1 ++;
}
else if (coin1>coin){
coin = coin1;
coin1=0;
}
}
for (int i = 0; i <list.size() ; i++) {
if (list.get(i)== list.get(i+1)){
coin1 ++;
}
if (list.get(i)== list.get(i+1)&&coin1 == coin ){
list2.add(list.get(i));
}
}
inputStream.close();
for (int i = 0; i < list2.size() ; i++) {
System.out.print(list2.get(i));
}
}
}