addChild(character, isCompleteWord = false) {
    if (!this.children.has(character)) {
      this.children.set(character, new TrieNode(character, isCompleteWord));
    }

    const childNode = this.children.get(character);

    // In cases similar to adding "car" after "carpet" we need to mark "r" character as complete.
    childNode.isCompleteWord = childNode.isCompleteWord || isCompleteWord;

    return childNode;
  }
@param {string} character
@param {boolean} isCompleteWord
@return {TrieNode}