Binary Tree Level Order Traversal II LeetCode solution in C++

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class Solution {
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  void dft(TreeNode *root, int level, map<int, vector<int>, std::greater<int>> &levelVals) {
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    if (root == nullptr) return;
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    if (levelVals.find(level) == levelVals.end())
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      levelVals[level] = {root->val};
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    else
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      levelVals[level].push_back(root->val);
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    dft(root->left, level + 1, levelVals);
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    dft(root->right, level + 1, levelVals);
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  }
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 public:
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  vector<vector<int>> levelOrderBottom(TreeNode *root) {
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    map<int, vector<int>, std::greater<int>> levelVals;
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    dft(root, 0, levelVals);
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    vector<vector<int>> res;
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    for (const auto &[level, vals] : levelVals) res.push_back(vals);
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    return res;
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  }
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};