在Python中按级别顺序打印二叉树，不包括Left或Right

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

def print_tree_by_level(root):
    if not root:
        return

    queue = [root]  # 使用队列来存储待遍历的节点
    result = []  # 存储按级别顺序打印的结果

    while queue:
        level_size = len(queue)  # 当前层级的节点数量

        for _ in range(level_size):
            node = queue.pop(0)  # 弹出队列中的第一个节点
            result.append(node.val)  # 将节点值添加到结果中

            # 将当前节点的左右子节点加入队列
            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)

    return result

# 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)

result = print_tree_by_level(root)
print(result)  # 输出: [1, 2, 3, 4, 5, 6]