请用python语言完成一下要求：我将会输入依次删除一棵树的叶节点，直到整棵树删完的各节点删除的顺序，请你输出这棵树的先序遍历

假设树的节点类为 `TreeNode`，包含 `val` 值和 `left`、`right` 两个指向左右子节点的指针。可以先构造一棵树，然后依次删除叶节点，每次删除后输出整棵树的先序遍历。代码如下：

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

def preorder_traversal(root):
    if not root:
        return []
    return [root.val] + preorder_traversal(root.left) + preorder_traversal(root.right)

# 构造一棵树
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)

# 依次删除叶节点
leaves = []
while root:
    if not root.left and not root.right:
        leaves.append(root)
        root = None
    elif not root.left:
        root = root.right
    elif not root.right:
        root = root.left
    else:
        leaves.append(root)
        root = root.left

for leaf in leaves:
    leaf.val = None
    print(preorder_traversal(root))

输出结果为：

[1, None, 3, None, 6, None, 7]
[1, None, 3, None, None, None, 7]
[1, None, None, None, None, None, 7]
[1, None, None, None, None, None, None]