222. 完全二叉树的节点个数

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# 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:
        num = 0
        if root != None:
            num = 1
            num += self.countNodes(root.left)
            num += self.countNodes(root.right)
        return num