題目：

題解：

# 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 countNodes(self, root: Optional[TreeNode]) -> int:if root==None:return 0return self.countNodes(root.left)+self.countNodes(root.right)+1