Construct string from binary tree

class Solution {
public:
    void f(TreeNode* root, string &s){

        if(root==NULL)return;

        s = s + '(';
        s = s + to_string(root->val);
        if(root->left==NULL && root->right!=NULL){
            s = s+"()";
        }
        f(root->left,s);
        f(root->right,s);
        s= s+ ')';

    }
    string tree2str(TreeNode* root) {
        string s = "";

        f(root,s);
        s.pop_back();
        s.erase(s.begin());
        return s;

    }
};