#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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