问NameError:未定义名称“TreeNode”

EN

from typing import Optional
class Solution:
    def findLeaves(self,root: Optional[TreeNode]) -> list[list[int]]:
        def post_order(node):
            nonlocal lookup

            if not node:
                return 0
            left_depth = post_order(node.left)
            right_depth = post_order(node.right)
            curr_depth = max(left_depth,right_depth) + 1
            lookup(curr_depth).append(node.val)
            return curr_depth
        lookup = defaultdict(list)
        post_order(root)
        return lookup.values()

demo = Solution()
root = [1,2,3,4,5]

v = demo.findLeaves(root)
print(v)

Traceback (most recent call last):
  File "E:\pythonProject\LeetCode\366_find_leaf_nodes.py", line 2, in <module>
    class Solution:
  File "E:\pythonProject\LeetCode\366_find_leaf_nodes.py", line 3, in Solution
    def findLeaves(self,root: TreeNode) -> list[list[int]]:
NameError: name 'TreeNode' is not defined**