Project Euler > Problem 21 > Amicable numbers (Java Solution)

Problem:

Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).

If d(a) = b and d(b) = a, where a [≠] b, then a and b are an amicable pair and each of a and b are called amicable numbers.

For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.

Evaluate the sum of all the amicable numbers under 10000.

Solution:

31626

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

public interface EulerSolution{

public String run();

}

/* 
 * Solution to Project Euler problem 21
 * By Nayuki Minase
 * 
 * http://nayuki.eigenstate.org/page/project-euler-solutions
 * https://github.com/nayuki/Project-Euler-solutions
 */


public final class p021 implements EulerSolution {
 
 public static void main(String[] args) {
  System.out.println(new p021().run());
 }
 
 
 public String run() {
  int sum = 0;
  for (int i = 1; i < 10000; i++) {
   if (isAmicable(i))
    sum += i;
  }
  return Integer.toString(sum);
 }
 
 
 private static boolean isAmicable(int n) {
  int m = divisorSum(n);
  return m != n && divisorSum(m) == n;
 }
 
 
 private static int divisorSum(int n) {
  int sum = 0;
  for (int i = 1; i < n; i++) {
   if (n % i == 0)
    sum += i;
  }
  return sum;
 }
 
}