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  1. using System;
  2. using System.Numerics;
  3. using System.Collections.Generic;
  4.  
  5. public static class BigIntExtensions
  6. {
  7.     public static BigInteger Factorial(this BigInteger integer)
  8.     {
  9.         if(integer < 1) return new BigInteger(1);
  10.  
  11.         BigInteger result = integer;
  12.         for (BigInteger i = 1; i < integer; i++)
  13.         {
  14.             result = result * i;
  15.         }
  16.  
  17.         return result;
  18.     }
  19. }
  20.  
  21. public class Test
  22. {
  23.     public static BigInteger CalcCount(BigInteger n, BigInteger r)
  24.     {
  25.         BigInteger result = n.Factorial() / (n - r).Factorial();
  26.         return result;
  27.     }
  28.  
  29.     public static int[] nthPerm(BigInteger myIndex, int n, int r, BigInteger total)
  30.     {
  31.         int j = 0, n1 = n;
  32.         BigInteger temp, index1 = myIndex;
  33.  
  34.         temp = total ;
  35.         List<int> indexList = new List<int>();
  36.  
  37.         for (int k = 0; k < n; k++) {
  38.                indexList.Add(k);
  39.         }
  40.  
  41.         int[] res = new int[r];
  42.  
  43.         for (int k = 0; k < r; k++, n1--) {
  44.             temp /= n1;
  45.             j = (int) (index1 / temp);
  46.             res[k] = indexList[j];
  47.             index1 -= (temp *  j);
  48.             indexList.RemoveAt(j);
  49.         }
  50.  
  51.         return res;
  52.     }
  53.  
  54.     public static void Main()
  55.     {
  56.         int[] testSet = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21};
  57.         BigInteger numPerms, n, r;
  58.         n = testSet.Length;
  59.         r = testSet.Length;
  60.         numPerms = CalcCount(n, r);
  61.         Console.WriteLine(numPerms);
  62.  
  63.         BigInteger testIndex = new BigInteger(1234567890987654321);
  64.         int[] myNthIndex = nthPerm(testIndex, (int) n, (int) r, numPerms);
  65.         int[] myNthPerm = new int[(int) r];
  66.  
  67.         for (int i = 0; i < (int) r; i++) {
  68.                myNthPerm[i] = testSet[myNthIndex[i]];
  69.         }
  70.  
  71.         Console.WriteLine(string.Join(",", myNthPerm));
  72.     }
  73. }
Success #stdin #stdout 0.04s 16220KB
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stdin
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Standard input is empty
stdout
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51090942171709440000
1,12,4,18,20,19,7,5,16,11,6,8,21,15,13,2,14,9,10,17,3
https://ideone.com/gU7A6e
language:
C# (gmcs 5.20.1)
created:
7 years ago
visibility:
public

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