Не пойму.
package com.javarush.task.task15.task1531;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigDecimal;
import java.math.BigInteger;
/*
Факториал
*/
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
int input = Integer.parseInt(reader.readLine());
reader.close();
System.out.println(factorial(input));
}
public static String factorial(int n) {
//add your code here
if (n < 0) return "0";
long fact = 1;
BigInteger bi = BigInteger.ONE;
for (int i = 1; i <= n; i++) {
bi = bi.multiply(BigInteger.valueOf(i));
}
return n + "! = " + String.valueOf(bi) ;
}
}