}
    return this;
  }

  /**
   * Convert this builder into an array of char[]s where the maximum index is the value of the
   * highest character that has been seen. The array will be sparse in the sense that any unseen
   * index will default to null.
   *
   * @return a "sparse" array that holds the replacement mappings.
   */
  public char[] @Nullable [] toArray() {
    char[][] result = new char[max + 1][];