Beautiful Mizuki!

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using System;
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using System.Linq;
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namespace ifmultiple3
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{
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    class Program
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    {
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        static long input = 0;
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        static long ladd = 0;
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        static void Add()
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        {
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            string numberString = Math.Abs(input).ToString();
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            long[] numbers = numberString.Select(c => (long)char.GetNumericValue(c)).ToArray();
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            foreach (long num in numbers)
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            {
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                ladd += num;
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            }
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        }
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        static void Main(string[] args)
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        {
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            try
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            {
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                if (args.GetLength(0) > 0)
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                {
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                    input = long.Parse(args[0]);
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                }
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                else
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                {
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                    Console.WriteLine("请输入数字");
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                    string s = Console.ReadLine();
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                    if (s != "")
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                    {
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                        input = long.Parse(s);
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                    }
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                    else
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                    {
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                        Console.WriteLine("输入不能为空");
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                        Console.ReadKey();
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                        return;
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                    }
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                }
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                long input1 = input;
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                while (true)
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                {
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                    if (input == 0)
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                    {
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                        Console.WriteLine(input1 + "是3的倍数");
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                        Console.ReadKey();
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                        break;
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                    }
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                    Add();
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                    if (ladd > 9)
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                    {
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                        input = ladd;
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                        ladd = 0;
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                    }
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                    else
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                    {
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                        if (ladd == 3 || ladd == 6 || ladd == 9)
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                        {
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                            Console.WriteLine(input1 + "是3的倍数");
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                            Console.ReadKey();
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                            break;
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                        }
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                        else
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                        {
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                            Console.WriteLine(input1 + "不是3的倍数");
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                            Console.ReadKey();
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                            break;
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                        }
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                    }
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                }
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            }
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            catch
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            {
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                Console.WriteLine("输入的不是数字或输入的是小数");
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                Console.ReadKey();
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            }
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        }
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    }
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}

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请输入数字
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123
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123是3的倍数