project-euler/HighlyDivisibleTriangularNumber.java

/* 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? */

import java.util.ArrayList;

public class HighlyDivisibleTriangularNumber {
  private static ArrayList<Long> divisors = new ArrayList<Long>();

  public static ArrayList<Long> getAllTriangleNumDivisors(long triangleNum) {
    // To contain all the divisors for the specified triangle number
    ArrayList<Long> divisors = new ArrayList<>();
    // To find all the divisors for the specified triangle number
    for (long i = 1; i <= triangleNum; i++) {
      if (triangleNum % i == 0)
        divisors.add(i);
    }
    // To return the ArrayList of divisors for the specified triangle number
    return divisors;
  }

  public static long calculateTriangleNumber() {
    long num = 0;
    long iterator = 0;

    while (divisors.size() <= 500) {
      iterator++;
      num += iterator;

      divisors = getAllTriangleNumDivisors(num);
    }

    return num;
  }
}