Find Duplicate Subtrees LeetCode solution in C++

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class Solution {
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 public:
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  unordered_map<string, int> dp;
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  vector<TreeNode *> ans;
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  string solve(TreeNode *root) {
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    if (!root) return "";
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    string left = solve(root->left);
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    string right = solve(root->right);
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    string code = to_string(root->val) + " " + left + " " + right;
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    if (dp[code] == 1) ans.push_back(root);
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    dp[code]++;
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    return code;
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  }
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  vector<TreeNode *> findDuplicateSubtrees(TreeNode *root) {
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    string dummy = solve(root);
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    return ans;
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  }
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};