// is allowed to return null when result is empty
static int[] intersectSortedIntArrays(int[] a, int[] b) {
  int i = 0, j = 0, la = l(a), lb = l(b);
  if (la == 0 || lb == 0) null;
  IntBuffer buf = new(max(la, lb));
  while (i < la && j < lb) {
    int x = a[i], y = b[j];
    if (x == y) {
      buf.add(x);
      ++i;
      ++j;
    } else if (x < y)
      ++i;
    else
      ++j;
  }
  ret buf.toArray();
}