#include <stdbool.h>
    #include <stdio.h>
 
    bool isUnique(const char x[], const 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(const char a[], char b[], const int x, const 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)
    {
       const char a[] = { '1', '2', '3', '4' };
       char b[] = { '\0', '\0', '\0', '\0', '\0' };
 
       perm(a, b, 4, 0);
    }

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