Project Euler > Problem 12 > Highly divisible triangular number (Java Solution)

Problem:

The sequence of triangle numbers is generated by adding the natural numbers. So the 7th triangle number would be 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28. The first ten terms would be:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...

Let us list the factors of the first seven triangle numbers:

1: 1
3: 1,3
6: 1,2,3,6
10: 1,2,5,10
15: 1,3,5,15
21: 1,3,7,21
28: 1,2,4,7,14,28

We can see that 28 is the first triangle number to have over five divisors.

What is the value of the first triangle number to have over five hundred divisors?

Solution:

76576500

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

public interface EulerSolution{

public String run();

}

/* 
 * Solution to Project Euler problem 12
 * By Nayuki Minase
 * 
 * http://nayuki.eigenstate.org/page/project-euler-solutions
 * https://github.com/nayuki/Project-Euler-solutions
 */


public final class p012 implements EulerSolution {
 
 public static void main(String[] args) {
  System.out.println(new p012().run());
 }
 
 
 public String run() {
  int num = 0;
  for (int i = 1; ; i++) {
   num += i;  // num is triangle number i
   if (countDivisors(num) > 500)
    return Integer.toString(num);
  }
 }
 
 
 private static int countDivisors(int n) {
  int count = 0;
  int end = Library.sqrt(n);
  for (int i = 1; i < end; i++) {
   if (n % i == 0)
    count += 2;
  }
  if (end * end == n)  // Perfect square
   count++;
  return count;
 }
 
}