#include <stdbool.h>
#include <stdio.h>
 
bool isUnique(char x[], int y)
{
   for (int i = 0; i < (y-1); i++)
     for (int j = i+1; j < y; j++)
       if (x[i] == x[j])
          return false;
 
   return true;
}
void perm(char a[], char b[], int x, int y)
{
   if ((y == x) && isUnique(b,x))
   {
     b[y] = '\0';
     printf("%s\n", b);
   }
   else
   {
     for (int i = 0; i < x; i++)
     {
        b[y] = a[i];
        if (y < x)
          perm(a,b,x,y+1);
     }
   }
}
int main(void)
{
   char a[] = {'1','2','3','4'};
   char b[] = {'\0','\0','\0','\0','\0'};
   perm(a,b,4,0);
   return 0;
}

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