Construct String from Binary Tree LeetCode solution in C++

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class Solution {
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  void tree2str(TreeNode *root, string &s) {
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    if (!root) return;
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    s += to_string(root->val);
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    if (!root->left && !root->right) return;
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    s.push_back('(');
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    tree2str(root->left, s);
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    s.push_back(')');
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    if (root->right) {
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      s.push_back('(');
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      tree2str(root->right, s);
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      s.push_back(')');
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    }
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  }
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 public:
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  string tree2str(TreeNode *root) {
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    string ans = "";
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    tree2str(root, ans);
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    return ans;
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  }
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};