Add floordiv and ceildiv for positives and negatives

[LuchoBazz]

// Reference: https://codeforces.com/contest/1782/submission/230743928
int floordiv(int a, int b) {
  return a > 0 ? a / b : -((-a + b - 1) / b);
}
 
int ceildiv(int a, int b) {
  return a > 0 ? (a + b - 1) / b : -(-a / b);
}