Largest Prime Factor.md

Even Fibonacci Numbers

Question

The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143?

Answer

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long num = sc.nextLong();
        ArrayList<Long> arr = new ArrayList<>();
        long f_num = (long) Math.floor(Math.sqrt(num));
        System.out.println(f_num);
        for (long i = 1; i <= f_num; i++) {
            if (num % i == 0) {
                arr.add(i);
                arr.add(num / i);
            }
        }

        ArrayList<Long> factors = new ArrayList<>();
        long largestPrime = 0;

        for (int i = 0; i < arr.size(); i++) {
            if (isPrime(arr.get(i))) {
                factors.add(arr.get(i));
                if (arr.get(i) > largestPrime) {
                    largestPrime = arr.get(i);
                }
            }
        }

        if (!factors.isEmpty()) {
            System.out.println("Prime Factors in the ArrayList: " + factors);
            System.out.println("Largest Prime Factor: " + largestPrime);
        } else {
            System.out.println("No prime factors found.");
        }
    }

    public static boolean isPrime(long num) {
        if (num <= 1) {
            return false;
        }
        if (num <= 3) {
            return true;
        }
        if (num % 2 == 0 || num % 3 == 0) {
            return false;
        }
        for (long i = 5; i * i <= num; i += 6) {
            if (num % i == 0 || num % (i + 2) == 0) {
                return false;
            }
        }
        return true;
    }
}