// Return any index of key in a sorted array A, or -1 if absent.

int BinarySearch(int[] A, int key, int low, int high) {
  if (low > high) { return -1; };
  int mid = (low + high) / 2;
  if (A[mid] == key) { return mid; };
  if (A[mid] > key) { return BinarySearch(A, key, low, mid - 1); };
  return BinarySearch(A, key, mid + 1, high);
}