C# 高精度求幂 poj1001

public static char[] exponentiation(string a,int r)
        {
            char[] aa = new char[1];
            string b = "";
            string c = a;
            for (int i = 0; i < r-1; i++)
            {
                aa = acm.Quadrature(c, a);
                b = "";
                foreach (var item in aa)
                {
                    b += item;
                }
                c = b;
            }
            return aa;
        }