struct TreeNode{    TreeNode * left, *right;    string val;    TreeNode(string i) :val(i), left(NULL), right(NULL){}}; bool IsOperator(const string & c){    if (c.length() > 1 || (c[0] >= ‘0‘ && c[0] <= ‘9‘))    {        return false;    }    return true;} string ExpressionTreeToString(TreeNode * node){    if (node == NULL)        return "";    string ret = node->val;    if (IsOperator(node->val))    {        if (node->left)        {            if (IsOperator(node->left->val))            {                ret.insert(0, "(" + ExpressionTreeToString(node->left) + ")");            }            else            {                ret.insert(0, ExpressionTreeToString(node->left));            }        }        if (node->right){            if (IsOperator(node->right->val))            {                ret += "(" + ExpressionTreeToString(node->right) + ")";            }            else            {                ret += ExpressionTreeToString(node->right);            }        }    }    return ret;}