#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <assert.h>

// LINKED LIST

typedef struct _Node {
    int value;
    struct _Node* next;
} Node;

void append(int value, Node** a_head, Node** a_tail) {
    // Create (allocate and initialize) the new node, which will become the tail.
    Node* new_tail = malloc(sizeof(*new_tail));
    new_tail -> value = value;  // same as `(*new_tail).value = value`
    new_tail -> next  = NULL;
    
    if(*a_tail == NULL) {  // if list is empty
        // List is empty, so head will also be tail.
        *a_head = new_tail;
    }
    else {   // List is not empty, so connect new_tail to the old tail.
        (*a_tail) -> next = new_tail;
    }

    // Connect the tail of the list to new_tail.
    *a_tail = new_tail;

    assert(*a_head != NULL && *a_tail != NULL);  // Sanity check: list is not empty anymore
}

void destroy_list(Node** a_head, Node** a_tail) {
    while(*a_head != NULL) { // as long as list is not empty
        free(*a_head);
        *a_head = (*a_head) -> next;  // WRONG - can't access *a_head anymore
    }
}

int main(int argc, char* argv[]) {
    // size==0
    Node* head = NULL;
    Node* tail = NULL;  // You don't always need to keep track of the tail.  It helps here.

    // size==1
    append(10, &head, &tail); // Pass address of head and tail so they can be modified.
    assert(head != NULL && tail != NULL); // Sanity check: list is no longer empty
    assert(head == tail);                 // Sanity check: head and tail are same (size==1)
    assert(head -> value == 10);          // Sanity check: head's value is 10
    
    // size==2
    append(11, &head, &tail);
    assert(head != tail);                 // Sanity check: head and tail are same (size==1)
    assert(head -> value == 10);          // Sanity check: head's value is 10
    assert(tail -> value == 11);          // Sanity check: tail's value is 11
    assert(tail -> next  == NULL);

    destroy_list(&head, &tail);
    
    return EXIT_SUCCESS;
}
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