自定义截取数，截取字符串，返回字符串数组。

 static void Main(string[] args)
        {
            string d = "努力的人最有魅力";
            var o= reback(d,3);
            Console.ReadLine();
        }
        public static List<string>  reback(string str,int first)
        {
            String shortstr=string.Empty, longstr = string.Empty;
            List<string> arr = new List<string>();
            if (string.IsNullOrWhiteSpace(str))
            {
                return arr;
            }else
            {
                var len = str.Length;
                var intAv = str.Length / first;
                if(len % first != 0){
                    intAv = intAv + 1;
                }
                var intC = first;
                for (int i = 0; i < intAv; i++)
                {
                    if(i == intAv-1||intC <= len)
                    {
                        if (i != 0)
                        {
                            if (i == intAv-1)
                            {
                                var o = str.Substring(intC, len -intC);
                                if (!string.IsNullOrWhiteSpace(o))
                                {
                                    arr.Add(o);
                                }
                            }
                            else
                            {
                                arr.Add(str.Substring(intC, first));
                            }
                            intC = intC+first;
                        }
                        else
                        {
                            arr.Add(str.Substring(0, first));
                        }
                    }
                  
                }
            }
            return arr;
        }