Logic to check the number is divisible by 3 or not?

Question

without using %, / or * , I have to find the no. is divisible by 3 or not?

it might be an interview question.

Thanks.

Answer 1

A number is divisible by three if its binary alternating digit sum is zero:

bool by3(int n) {
  int s=0;
  for (int q=1; n; s+=q*(n&1), n>>=1, q*=-1);
  return !s;
}