Python实现LeetCode 814二叉树剪枝的问题解惑

我在解决LeetCode第814题「Binary Tree Pruning（二叉树剪枝）」时遇到了问题，题目要求返回一棵修改后的二叉树，移除所有不包含值1的子树。

以下是我的初始代码：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def pruneTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        if root is None:
            return None

        self.pruneTree(root.left)
        self.pruneTree(root.right)

        if root.val == 0 and root.left is None and root.right is None:
            root = None
            return 

        return root

遗憾的是，这段代码没有得到预期结果，运行出现错误。之后我得到了一段可以正常运行的代码：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def pruneTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        if root is None:
            return None

        root.left = self.pruneTree(root.left)
        root.right = self.pruneTree(root.right)

        if root.val == 0 and root.left is None and root.right is None:
            root = None
            return 

        return root

我无法理解为何未对root.left和root.right进行赋值的初始代码无法正常工作，而修改后的赋值版本可以。希望有人能解释其中的底层逻辑问题，帮助我理解。

内容的提问来源于stack exchange，提问作者user22222314