Construct String from Binary Tree LeetCode solution in Python

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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('('); tree2str(root->left,s); s.push_back(')');
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        if(root->right){
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            s.push_back('('); tree2str(root->right,s); 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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};