/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     string val；
 *     TreeNode *left;
 *     TreeNode *right;
 * };
 */
class Solution {
public:


    void dfs (TreeNode *root , string &res)
    {
        if(!root) return;
        if(root->left == NULL && root->right == NULL)
        {
            res += root -> val;
        }
        else
        {
            res += '(';
            dfs(root->left ,res );
            res += root->val;
            dfs(root->right , res);
            res += ')';
        }

    }

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

        return res;
    }
};

根节点另外拿出来