Find the largest prime number factor?

Question

I need to find The prime factors of 13195 are 5, 7, 13 and 29. / * Largest is 377. * / What is the largest prime factor of the number 600851475143 ?

#include<         


        

Answer 1

public class LargeFactor{

     public static void main(String []args){
     long num = 600851475143L;
     long largestFact = 0;
     long[] factors = new long[2];

    for (long i = 2; i * i < num; i++) {
         if (num % i == 0) { // It is a divisor
        factors[0] = i;
        factors[1] = num / i;

        for (int k = 0; k < 2; k++) {
            boolean isPrime = true;
            for (long j = 2; j * j <  factors[k]; j++) {
                if (factors[k] % j == 0) {
                    isPrime = false;
                    break;
                 }
             }
             if (isPrime && factors[k] > largestFact) {
                largestFact = factors[k];
             }
            }
        }
     }
     System.out.println(largestFact);
}
}

Above code utilises the fact that we only need to check all numbers up to the square root when looking for factors.