如何以格式良好的方式打印出树？

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def print_tree(root):
    if not root:
        return

    # 获取树的高度
    height = get_height(root)

    # 初始化结果列表
    result = [[''] * (2 ** height - 1) for _ in range(height)]

    # 填充结果列表
    fill_result(root, result, 0, 0, len(result[0]) - 1)

    # 打印结果列表
    for row in result:
        print(' '.join(row))

def get_height(node):
    if not node:
        return 0
    return max(get_height(node.left), get_height(node.right)) + 1

def fill_result(node, result, row, left, right):
    if not node:
        return

    # 计算当前节点在结果列表中的列索引
    mid = (left + right) // 2

    # 将当前节点的值填入结果列表
    result[row][mid] = str(node.val)

    # 递归填充左子树和右子树
    fill_result(node.left, result, row + 1, left, mid - 1)
    fill_result(node.right, result, row + 1, mid + 1, right)

# 构造一个示例树
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)
root.right.left = TreeNode(6)
root.right.right = TreeNode(7)

# 打印树的结构
print_tree(root)

      1
   2     3
 4   5 6   7