如何打印二叉树中已添加到队列的节点值？

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

def printTreeValues(root):
    if not root:
        return
    
    queue = [root]  # 创建队列并将根节点添加到队列中

    while queue:
        node = queue.pop(0)  # 取出队列中的节点
        print(node.val)  # 打印节点的值

        if node.left:
            queue.append(node.left)  # 将左子节点添加到队列中
        if node.right:
            queue.append(node.right)  # 将右子节点添加到队列中

# 创建一个二叉树
#        1
#       / \
#      2   3
#     / \   \
#    4   5   6
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)
root.right.right = TreeNode(6)

printTreeValues(root)  # 打印二叉树中已添加到队列的节点值