content: /**
   * File: tree_node.dart
   * Created Time: 2023-2-12
   * Author: Jefferson (JeffersonHuang77@gmail.com)
   */
  
  /* 二叉树节点类 */
  class TreeNode {
    int val; // 节点值
    int height; // 节点高度
    TreeNode? left; // 左子节点引用
    TreeNode? right; // 右子节点引用
  
    /* 构造方法 */
    TreeNode(this.val, [this.height = 0, this.left, this.right]);
  }
  
  /* 将列表反序列化为二叉树：递归 */
  TreeNode? listToTreeDFS(List<int?> arr, int i) {
    if (i < 0 || i >= arr.length || arr[i] == null) {
      return null;
    }
    TreeNode? root = TreeNode(arr[i]!);
    root.left = listToTreeDFS(arr, 2 * i + 1);
    root.right = listToTreeDFS(arr, 2 * i + 2);
    return root;
  }
  
  /* 将列表反序列化为二叉树 */
  TreeNode? listToTree(List<int?> arr) {
    return listToTreeDFS(arr, 0);
  }
  
  /* 将二叉树序列化为列表：递归 */
  void treeToListDFS(TreeNode? root, int i, List<int?> res) {
    if (root == null) return;
    while (i >= res.length) {
      res.add(null);
    }
    res[i] = root.val;
    treeToListDFS(root.left, 2 * i + 1, res);
    treeToListDFS(root.right, 2 * i + 2, res);
  }
  
  /* 将二叉树序列化为列表 */
  List<int?> treeToList(TreeNode? root) {
    List<int?> res = [];
    treeToListDFS(root, 0, res);
    return res;
  }
  