largest_prime_factor.cpp

#include <iostream>
#include <math.h>

using namespace std;

long findLargestPrimeFactor(long n) {
    long squareRootOfN = (long) ceil(sqrt(n));
    long largestPrimeFactor = -1;

    for (long i = 2; i <= squareRootOfN; i++) {
        if (n % i == 0) {
            long test = findLargestPrimeFactor(n / i);
            if (test > largestPrimeFactor) {
                largestPrimeFactor = test;
            }
        }
    }

    return (largestPrimeFactor != -1) ? largestPrimeFactor : n;
}

int main() {
    cout << findLargestPrimeFactor(600851475143) << endl;
    return 0;
}