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vector<int> largestValues(TreeNode* root) {
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    vector<int> res;
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    if (root == nullptr) return res;
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    queue<TreeNode *> q;
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    q.push(root);
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    while(!q.empty()) {
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        int max_v = INT_MIN;
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        for(int i = 0, size = q.size(); i < size; i++) {
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            root = q.front(); q.pop();
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            max_v = max(max_v, root->val);
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            if (root->left) q.push(root->left);
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            if (root->right) q.push(root->right);
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        }
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        res.push_back(max_v);
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    }
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    return res;
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}