如何在Dart中打印二叉树图？

class TreeNode {
  int val;
  TreeNode left;
  TreeNode right;

  TreeNode(this.val, [this.left, this.right]);
}

void printBinaryTree(TreeNode root, [int level = 0, String branch = '']) {
  if (root == null) {
    return;
  }

  final indent = ' ' * level;
  print('$indent$branch${root.val}');

  if (root.left != null) {
    printBinaryTree(root.left, level + 1, '├─左: ');
  }

  if (root.right != null) {
    printBinaryTree(root.right, level + 1, '└─右: ');
  }
}

void main() {
  // 构造一个二叉树
  final root = TreeNode(1);
  final node1 = TreeNode(2);
  final node2 = TreeNode(3);
  final node3 = TreeNode(4);
  final node4 = TreeNode(5);
  final node5 = TreeNode(6);

  root.left = node1;
  root.right = node2;
  node1.left = node3;
  node1.right = node4;
  node2.right = node5;

  // 打印二叉树
  printBinaryTree(root);
}

1
├─左: 2
│     ├─左: 4
│     └─右: 5
└─右: 3
      └─右: 6