getCharSet_() {
    this.charSet_ = [];
    for (let i = 0; i < this.textLen_; ++i) {
      if (this.charSet_.indexOf(this.textString_[i]) === -1) {
        this.charSet_.push(this.textString_[i]);
      }
    }
    this.charSetSize_ = this.charSet_.length;
  }
Get the set of unique characters from text.