flat

  getFlatTree(): TreeNode[] {
    return this.flatTreeRecursive(this._root);
  }

  private flatTreeRecursive(root: TreeNode): TreeNode[] {
    let res = [];
    for (let c of root.children) {
      res.push(c);
      res = res.concat(this.flatTreeRecursive(c));
    }
    return res;
  }