Problem12.java

package com.javamultiplex.projecteuler;

import java.util.ArrayList;
import java.util.List;

/**
 * 
 * @author Rohit Agarwal
 * @category Project Euler Problems
 * @Problem 12 - Highly divisible triangular number.
 *
 */
public class Problem12 {

	public static void main(String[] args) {

		int target = 500;
		long number = 1;
		long triangularNumber = 0;
		int count = 0;
		while (true) {
			/*
			 * Step 1: Find the triangular number.
			 * 
			 * As triangular number is generated by adding natural numbers so
			 * Sum of first n natural number is = n(n+1)/2
			 * 
			 */
			triangularNumber = (number) * (number + 1) / 2;
			// Step 2: Get the number of divisors.
			count = getCountofDivisors(triangularNumber);
			// Step 3: If number of divisors > 500 then print it otherwise continue.
			if (count > target) {
				System.out.println("The value of the first triangle number to have over five hundred divisors :"
						+ triangularNumber);
				break;
			}
			number++;
		}

	}

	private static int getCountofDivisors(long triangularNumber) {

		long limit = (long) Math.sqrt(triangularNumber);
		List<Long> list = new ArrayList<>();
		long temp = 0;
		for (long i = 1; i <= limit; i++) {
			if (triangularNumber % i == 0) {
				list.add(i);
				temp = triangularNumber / i;
				if (i != temp) {
					list.add(temp);
				}
			}
		}
		int divisors = list.size();
		return divisors;
	}

}