Cracking Coding Interview - 10.5 Sparse Search

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Input:  ball, {"at", "", "", "", "ball", "", "", "car", "", "", "dad", "", ""}
Output: 4

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public int sparseSearch(String[] array, String str) {
  return sparseSearch(array, str, 0, array.length);
}

private int sparseSearch(String[] array, String str, int start, int end) {
  if (start > end) {
    return -1;
  }
  int mid = (start + end) / 2;
  if (array[mid] == "") {
    int newMid = leftNonEmpty(array, mid);
    if (newMid == -1) {
      newMid = rightNonEmpty(array, mid);    
    }
    if (newMid == -1) {
      // 说明左右两边都是空串
      return -1;
    }
    mid = newMid;
  }
  
  if (array[mid] == str) {
    return mid;
  }
  if (array[mid] < str) {
    return sparseSearch(array, str, mid + 1, end);
  }
  return sparseSearch(array, str, start, mid - 1);
}

private int leftNonEmpty(String[] array, int mid, int start) {
  while (mid >= start) {
    if (array[mid] != "") {
      return mid;
    }
    mid--;
  }
  return -1;
}

private int rightNonEmpty(String[] array, int mid, int end) {
  while (mid <= end) {
    if (array[mid] != "") {
      return mid;
    }
    mid++;
  }
  return -1;
}