class Solution:
    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
        
        # -- VERY good for a review!
        # -- mine 
        res = [0]
        def dfs(node):
            if not node:
                return 0
            
            left = dfs(node.left)
            right = dfs(node.right)

            res[0] = max(left+right, res[0])
            return 1 + max(left, right) # Don't forget +1 for current node!

        dfs(root)
        return res[0]