Project Euler > Problem 120 > Square remainders (Java Solution)

Problem:

Let r be the remainder when (a[−]1)n + (a+1)n is divided by a2.

For example, if a = 7 and n = 3, then r = 42: 63 + 83 = 728 [≡] 42 mod 49. And as n varies, so too will r, but for a = 7 it turns out that rmax = 42.

For 3 [≤] a [≤] 1000, find [∑] rmax.

Solution:

648

Code:
The solution may include methods that will be found here: Library.java .

public interface EulerSolution{

public String run();

}

/* 
 * Solution to Project Euler problem 20
 * By Nayuki Minase
 * 
 * http://nayuki.eigenstate.org/page/project-euler-solutions
 * https://github.com/nayuki/Project-Euler-solutions
 */


public final class p020 implements EulerSolution {
	
	public static void main(String[] args) {
		System.out.println(new p020().run());
	}
	
	
	public String run() {
		String temp = Library.factorial(100).toString();
		int sum = 0;
		for (int i = 0; i < temp.length(); i++)
			sum += temp.charAt(i) - '0';
		return Integer.toString(sum);
	}
	
}