Check if a Number is Prime?

bool IsPrime(int n)

if (n <= 1) return false;

if (n <= 3) return true;

if (n % 2 == 0 || n % 3 == 0) return false;

for (int i = 5; i * i <= n; i += 6)

if (n % i == 0 || n % (i + 2) == 0)

return false;

Follow on:

return true;

Explanation:

Check divisibility by 2, 3, then test possible divisors of form 6k ± 1 up to √n.