Project Euler > Problem 70 > Totient permutation (Java Solution)

Problem:

Euler's Totient function, φ(n) [sometimes called the phi function], is used to determine the number of positive numbers less than or equal to n which are relatively prime to n. For example, as 1, 2, 4, 5, 7, and 8, are all less than nine and relatively prime to nine, φ(9)=6.
The number 1 is considered to be relatively prime to every positive number, so φ(1)=1.

Interestingly, φ(87109)=79180, and it can be seen that 87109 is a permutation of 79180.

Find the value of n, 1 [<] n [<] 107, for which φ(n) is a permutation of n and the ratio n/φ(n) produces a minimum.

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);
	}
	
}