/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     string val；
 *     TreeNode *left;
 *     TreeNode *right;
 * };
 */
class Solution {
public:
    string res;
    void dfs(TreeNode *root){
        if(!root) return;
        if(!root->right && !root->left){
            res += root->val;
        }else{
        res += '(';
        dfs(root->left);
        res += root->val;
        dfs(root->right);
        res += ')'; 
        }

    }

    string expressionTree(TreeNode* root) {
        dfs(root->left); res += root->val;  dfs(root->right);
        return res;
    }
};