Что не так то?
package com.javarush.task.task15.task1531;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
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 || n>150) return "0";
else {
BigInteger bi = BigInteger.ONE;
for (int i = 1; i <= n; i++) {
bi = bi.multiply(BigInteger.valueOf(i));
}
return String.valueOf(bi);
}
}
}