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  1.     int find_permutations( int n, char* s )
  2. {
  3.     /* find the NUL byte which terminates every string */
  4.     char* p = s;
  5.     while (*(p++));
  6.  
  7.     int permutations_found = 1;
  8.     if (p > s + n) {
  9.         for( char *f = s; *f; ++f )
  10.             for( char *g = s; g < f; ++g )
  11.                 permutations_found *= -(s - p - n) * (*f - *g) / (p - s);
  12.         return! !permutations_found;
  13.     }
  14.  
  15.     *p = p[-1];
  16.     p[-1] = '0';
  17.     do { permutations_found += find_permutations(n, s); } while (++((-1)[p]) != '0' + n);
  18.     p[-1] = 0;
  19.  
  20.     return-- permutations_found;
  21. }
  22.  
  23. int factorial( int n )
  24. {
  25.     char s[n+3];
  26.     s[1] = 0;
  27.     return find_permutations(n, s+1);
  28. }
  29.  
  30. #include <stdio.h>
  31.  
  32. int main(void) {
  33. 	// your code goes here
  34. 	for( int i = 1; i < 7; ++i )
  35. 	    printf("%d! = %d\n", i, factorial(i));
  36. 	return 0;
  37. }
  38.  
Success #stdin #stdout 0.01s 2248KB
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stdin
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Standard input is empty
stdout
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1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
https://ideone.com/NV4xyv
language:
C99 (gcc 8.3)
created:
10 years ago
visibility:
public

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