Just realized this fact, but need confirmation if I'm not missing anything (Although it got AC on a problem :P) because it looks simple AF!!
Every integer can be represented as one of the following-> 6k, 6k+1, 6k+2, 6k+3, 6k+4, 6k+5.
6k -> divisible by 6 (Hence not prime)
6k+1 -> Prime
6k+2 -> Or 2*(3k+1); Hence divisible by 2 (Not prime)
6k+3 -> Divisible by 3 (Not prime)
6k+4 -> Divisible by 2 (Not prime)
6k+5 -> Prime
Therefore, each prime number (>3) can be represented in either 6k+1 form or 6k+5 form.
bool isPrime(int x) {
if (x==1) return false;
if (x==2 || x==3) return true;
//If (x == 6k+1)
int k = (x-1)/6;
if (x == (6*k+1)) return true;
//If (x == 6k+5)
k = (x-5)/6;
return (x == (6*k+5));
}