tsaiwn perm% cat -n perm.c
     1	#include<stdio.h>
     2	/*List all permutations of n characters */
     3	/* Backtracking --- CopyLeft by tsaiwn@csie.nctu.edu.tw */
     4	/*  輸入 n 個不同字母然後印出該些字母的所有排列 */
     5	int val[99], id=-1, nPerm, n;  /* n characters */
     6	int getLetters();
     7	void visit(), writePerm();
     8	char letters[99];
     9	int main( ) {
    10	   while(38) {
    11	      n = getLetters();
    12	      if(n <= 0) break;
    13	      printf("n=%d\n", n);
    14	      id = -1; nPerm=0;
    15	      visit(0);
    16	      printf("===Total %d permutations.\n", nPerm);
    17	   } printf("\nBye!\n");
    18	}
    19	void visit(int k) {
    20	   int t;
    21	   val[k] = ++id;
    22	   /*  printf(" id=%d ", id);   /**/
    23	   if(id==n) writePerm();
    24	   for(t=0; t<n; ++t)
    25	      if(val[t] == 0) visit(t);
    26	   id--; val[k] = 0;
    27	}
    28	void writePerm( ) {
    29	   int i;
    30	   for(i=0; i<n; ++i) printf("%c ", letters[val[i] - 1]);
    31	   printf("\n"); nPerm++;
    32	}
    33	int getLetters( ) {
    34	   static char tmp[99], i, k;
    35	   printf("輸入字母群: ");
    36	   fgets(tmp, 99, stdin); if(feof(stdin)) return -1;
    37	   k=0;
    38	   for(i=0; i< strlen(tmp); ++i) {
    39	      if(tmp[i] == '\n') break; /* End Of Line */
    40	      if(tmp[i] == ' ') continue; /* ignore blank*/
    41	      val[k] = 0;
    42	      letters[k++] = tmp[i];
    43	   }
    44	   return k;
    45	}

tsaiwn perm% gcc perm.c
tsaiwn perm% ./a.out
輸入字母群: ab
n=2
a b 
b a 
===Total 2 permutations.
輸入字母群: ABC
n=3
A B C 
A C B 
B A C 
C A B 
B C A 
C B A 
===Total 6 permutations.
輸入字母群: 
Bye!

tsaiwn perm%