Most Frequent Subtree Sum LeetCode solution in Python

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# Definition for a binary tree node.
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# class TreeNode:
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#     def __init__(self, val=0, left=None, right=None):
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#         self.val = val
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#         self.left = left
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#         self.right = right
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class Solution:
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    def findFrequentTreeSum(self, root: Optional[TreeNode]) -> List[int]:
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        def dfs(root):
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            if not root:
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                return 0
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            l = dfs(root.left)
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            r = dfs(root.right)
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            res = root.val + l + r
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            d[res] += 1
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            return res
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        d = collections.Counter()
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        dfs(root)
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        maxi = max(d.values())
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        return [i for i in d if d[i] == maxi]
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# An Upvote will be encouraging