Most Frequent Subtree Sum LeetCode solution in C++

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class Solution {
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 private:
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  unordered_map<int, int> m;
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  int maxi = 0;
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  int f(TreeNode *root) {
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    if (!root) return 0;
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    int l = f(root->left);
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    int r = f(root->right);
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    int sum = root->val + l + r;
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    m[sum]++;
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    maxi = max(maxi, m[sum]);
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    return sum;
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  }
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 public:
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  vector<int> findFrequentTreeSum(TreeNode *root) {
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    vector<int> ans;
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    f(root);
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    for (auto &e : m) {
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      if (e.second == maxi) ans.push_back(e.first);
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    }
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    return ans;
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  }
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};