expression tree to string

+
/ \
1 *
/ \
5 6

这样的。要返回1+（5*6）

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;
}