problem24.1.c

problem24.1.c

#include <string.h>
#include <stdio.h>
#include <stdbool.h>

void swap(char a[], size_t i, size_t j) {
  char temp = a[i];
  a[i] = a[j];
  a[j] = temp;
}

bool swapped_before(char a[], size_t k, size_t i) {
    for (size_t j=k; j<i; j++) {
        if (a[j] == a[i]) {
            return true;
        }
    }
    return false;
}

/**
 * Fix a[0]..a[k - 1] but permute characters a[k]..a[len - 1]
 * Print out each permutation.
 *
 * @param[in,out] a The array to permute
 * @param[in]     n The size of the array
 * @param[in]     k The starting index at which we will permute
 *
 * @post The string a remains unchanged
 */
void permute(char a[], size_t n, size_t k) {
    if (k == n-1) {
        printf("%s\n", a);
        return;
    }
    for (size_t i = k; i < n; i+= 1) {
        if(!swapped_before(a, k, i)) {
            swap(a, k, i);
            permute(a, n, k+1);
            swap(a, i, k);
        }
    }
}

int main() {
    char str[] = "aaa";
    permute(str, strlen(str), 0);
}