如何打印二叉树？

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

def printBinaryTree(root):
    if root is None:
        return

    # 前序遍历
    print(root.val)
    printBinaryTree(root.left)
    printBinaryTree(root.right)

# 创建一个二叉树
#        1
#       / \
#      2   3
#     / \
#    4   5
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)

# 打印二叉树
printBinaryTree(root)