#include <stdio.h>

#define N 10

void print_perm(const char *a, int n) {
    for (int i = 0; i < n; i++) {
        putchar(a[i]);
    }
    putchar('\n');
}

// Heap's algorithm (iterative form)
void heap_permutations(char *a, int n) {
    int c[N] = {0};  // counters array
    print_perm(a, n);

    int i = 0;
    while (i < n) {
        if (c[i] < i) {
            // if i is even, swap a[0] and a[i]
            // else swap a[c[i]] and a[i]
            int swap_idx = (i % 2 == 0) ? 0 : c[i];
            char tmp = a[swap_idx];
            a[swap_idx] = a[i];
            a[i] = tmp;

            print_perm(a, n);

            c[i] += 1;
            i = 0;
        } else {
            c[i] = 0;
            i++;
        }
    }
}

int main(void) {
    char a[N] = {'a','b','c','d','e','f','g','h','i','j'};
    heap_permutations(a, N);
    return 0;
}