Question

I hate to have to ask, but I’m pretty stuck here.

I need to test a sequence of numbers to find the first which has over 500 factors: http://projecteuler.net/index.php?section=problems&id=12

-At first I attempted to brute force the answer (finding a number with 480 after a LONG time)

-I am now looking at determining the prime factors of a number and then use them to find all other factors.

I am currently at the stage where I can get an array of prime factors for any number I input – i.e 300 has the prime factors 2 2 3 5 5

Using this array of prime factors I need to be able to calculate the remaining factors – This is the part I am stuck on. Basically, as I understand it, I need to calculate ALL possible combinations of the numbers in the array…

i.e 2 * 2
2 * 2 * 3
2 * 2 * 3 * 5
2 * 3
2 * 3 * 3
…and so forth – But where it gets interesting is with things like…
2 * 5
2 * 3 * 5
…i.e Numbers which are not adjacent to each other in the array

I can’t think of a way to code this in a generic fashion for any length array…

I need help! P.S – I am working in Java

EDIT: My brute force code – As it has been suggested brute forcing the problem will work and so there may be an error in my code 🙁

package euler.problem12;  public class Solution {      public static void main(String[] args) {         int next = 1;         int triangle = 0;         int maxFactors = 0;          while(true) {             triangle = triangle + next;              int factors = 1;             int max = (int) triangle / 2;              for(int i = 1; i <= max; ++i) {                 if(triangle % i == 0) {                     factors ++;                 }             }              if(factors > maxFactors) {                 maxFactors = factors;                  System.out.println(triangle + '\t' + factors);             }              next++;         }     } } 

Answer 1

As far as I can tell, question 12 doesn’t mention anything about prime numbers? Is this the one you’re looking at?

The sequence of triangle numbers is generated by adding the natural numbers…

If so, then perhaps not thinking about primes will help? 😉