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  1. using System;
  2.  
  3. namespace Symbolic
  4. {
  5.     class Program
  6.     {
  7.         static char Char(int d)
  8.         {
  9.             return (char)(d + (int)'0');
  10.         }
  11.  
  12.         static int Digit(char c)
  13.         {
  14.             return (int)(c - '0');
  15.         }
  16.  
  17.         static string Multiply(string a, string b)
  18.         {
  19.             int[] acc = new int[a.Length + b.Length];
  20.  
  21.             for (int i = a.Length - 1; i >= 0; i--)
  22.             {
  23.                 int rest = 0;
  24.                 for (int j = b.Length - 1; j >= 0; j--)
  25.                 {
  26.                     int temp = acc[i + j + 1] + (Digit(a[i]) * Digit(b[j])) + rest;
  27.                     acc[i + j + 1] = temp % 10;
  28.                     rest = temp / 10;
  29.                 }
  30.                 acc[i] = acc[i] + rest;
  31.             }
  32.  
  33.             int start = 0;
  34.             while (acc[start] == 0)
  35.             {
  36.                 start++;
  37.             }
  38.  
  39.             char[] result = new char[acc.Length - start];
  40.             for (int i = start; i < acc.Length; i++)
  41.             {
  42.                 result[i - start] = Char(acc[i]);
  43.             }
  44.  
  45.             return new string(result);
  46.         }
  47.  
  48.         static string Previous(char c)
  49.         {
  50.             return Char(Digit(c) - 1).ToString();
  51.         }
  52.  
  53.         static string Decrement(string a)
  54.         {
  55.             string prev = Previous(a[0]);
  56.  
  57.             if (a.Length == 1)
  58.             {
  59.                 return prev;
  60.             }
  61.  
  62.             if (a[a.Length - 1] != '0')
  63.             {
  64.                 var rest = a.Substring(0, a.Length - 1);
  65.  
  66.                 return rest + Previous(a[1]);
  67.             }
  68.             else
  69.             {
  70.                 var rest = a.Substring(0, a.Length - 2);
  71.  
  72.                 if ((rest == "") && (prev == "0"))
  73.                     prev = "";
  74.  
  75.                 return rest + prev + '9';
  76.             }
  77.         }
  78.  
  79.         static string Fact(string value)
  80.         {
  81.             string result = "1";
  82.  
  83.             while (value != "0")
  84.             {
  85.                 result = Multiply(result, value);
  86.                 value = Decrement(value);
  87.             }
  88.  
  89.             return result;
  90.         }
  91.  
  92.         static void Main(string[] args)
  93.         {
  94.             Console.WriteLine(Fact("30"));
  95.         }
  96.     }
  97. }
  98.  
Success #stdin #stdout 0.02s 14764KB
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265252859812191058636308480000000
https://ideone.com/3rcJlH
language:
C# (gmcs 5.20.1)
created:
6 years ago
visibility:
public

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