C#版 大数计算器减法

class BigReduce : BigCalculate
 

   {
        public override string Oper(string num1, string num2)
        {
            bool isMinus = false;
            //先判断计算结果是否为正数(即num1是否大于num2)，如果相等，直接返回0
            //若结果为负数，则互换num1，num2，并在返回值前加'-'
            if (num1.Equals(num2))
            {
                return "0";
            }
            else if (Max(num1, num2).Equals(num2))//如果预计计算结果为负数，则将num1 num2调换
            {
                ChangeNum(ref num1, ref num2);
                isMinus = true;
            }

            #region MyRegion
            //获得较大数的位数
            int len = num1.Length > num2.Length ? num1.Length : num2.Length;
            //将较小数差位补0
            string tempNum1 = new string('0', len - num1.Length) + num1;
            string tempNum2 = new string('0', len - num2.Length) + num2;

            int flag = 0;//进位符
            List liCh = new List();//存放运算后数字
            for (int i = len - 1; i >= 0; i--)
            {
                if (tempNum1[i] >= tempNum2[i] + flag)
                {
                    liCh.Add((char)(tempNum1[i] - tempNum2[i] - flag + '0'));
                    flag = 0;
                }
                else
                {
                    liCh.Add((char)(tempNum1[i] - tempNum2[i] - flag + 10 + '0'));
                    flag = 1;
                }
            }

            //若结果高位有0，则去掉
            //若结果为负数，则返回结果加"-"
            if (isMinus == true)
            {
                return "-" + InvertOrder(new string(liCh.ToArray())).TrimStart('0');
            }
            return InvertOrder(new string(liCh.ToArray())).TrimStart('0');
            #endregion

        }
}