Using a for loop:

int firstBig(int i)
{
  for(int k = i; k != 0; k = next(k))
  {
    if(k > 100) return k;
  }
  return 0;
}

Using a while loop:

int firstBig(int i)
{
  int k = i;
  while(k != 0)
  {
    if(k > 100) return k;
    k = next(k);
  }
  return 0;
}