#include <stdio.h>#include <stdlib.h>char array[] = "abcd";#define N 4#define M 3int queue[N] = {0};int top = 0;int flag[N] = {0};  void perm(int s, int n) {   int i;    if (s > n)   {     return;   }    if (s == n)   {     for (i = 0; i < n; i++)     {       printf("%c", queue[i]);     }     printf("/t");     return ;   }    for (i = 0; i < n; i++)   {     if (flag[i] == 0)     {       flag[i] = 1;       queue[s] = array[i];       perm(s+1, n);       flag[i] = 0;     }   } }  void comb(int s, int n, int m) {   int i;    if (s > n)     return ;    if (top == m)   {     for (i = 0; i < m; i++)     {       printf("%c", queue[i]);     }     printf("/t");     return ;   }    queue[top++] = array[s];   comb(s+1, n, m);   top--;   comb(s+1, n, m);  }  int main() {   printf("/nperm():/n");   perm(0, N);   printf("/ncombination():/n");   comb(0, N, M);   printf("/n");   return 0; }

perm():abcd  abdc  acbd  acdb  adbc  adcb  bacd  badc  bcad  bcdabdac  bdca  cabd  cadb  cbad  cbda  cdab  cdba  dabc  dacbdbac  dbca  dcab  dcbacombination():abc   abd   acd   bcd