Count Complete Tree Nodes

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# Definition for a binary tree node.
# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val = 0, left = nil, right = nil)
#         @val = val
#         @left = left
#         @right = right
#     end
# end

# @param {TreeNode} root
# @return {Integer}
def count_nodes(root)     
  if root.nil?
      return 0
  end
  return 1 + count_nodes(root.left) + count_nodes(root.right)
end

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class Solution:
    def countNodes(self, root: Optional[TreeNode]) -> int:
        if not root:
            return 0

        return 1 + self.countNodes(root.left) + self.countNodes(root.right)