# 2020q3 Homework (quiz1)
contributed by < `chewei3` >

## add_entry

void add_entry(node_t **head, int new_value)
{
    node_t **indirect = head;

    node_t *new_node = malloc(sizeof(node_t));
    new_node->value = new_value;
    new_node->next = NULL;

    AA1;
    while (*indirect)
        indirect = &(*indirect)->next;
    AA2;
}

### AA1

    assert(new_node);

### AA2

    *indirect = new_node;

## swap_pair

node_t *swap_pair(node_t *head)
{
    for (node_t **node = &head; *node && (*node)->next; BB1) {
        node_t *tmp = *node;
        BB2;
        tmp->next = (*node)->next;
        (*node)->next = tmp;
    }
    return head;
}

### BB1

node = &(*node)->next->next

### BB2

*node = (*node)->next;

### swap_pair 的 pointer to pointer 版

void swap_pair(node_t **head) {
    for (; *head && (*head)->next; head = &(*head)->next->next) {
        node_t *tmp = *head;
        *head = (*head)->next;
        tmp->next = (*head)->next;
        (*head)->next = tmp;
    }
}

## reverse

node_t *reverse(node_t *head)
{
    node_t *cursor = NULL;
    while (head) {
        node_t *next = head->next;
        CCC;
        head = next;
    }
    return cursor;
}

### CCC

head->next = cursor;
cursor = head;

### reverse 的 pointer to pointer 版本

void reverse(node_t **head) {
    node_t *cursor = NULL;
    while (*head) {
      node_t *next = (*head)->next;
      (*head)->next = cursor;
      cursor = *head;
      *head = next;
    }
    *head = cursor;
}

### recursive reverse

void rev_recursive(node_t **head, node_t *cur) {
    if (!cur)
      return;
    if (!cur->next) {
      *head = cur;
      return;
    }
    rev_recursive(head, cur->next);
    cur->next->next = cur;
    cur->next = NULL;
}