typedef struct listNode {
    //指向前驅(qū)節(jié)點(diǎn)
    struct listNode *prev;
    //指向后繼節(jié)點(diǎn)
    struct listNode *next;
    //維護(hù)當(dāng)前節(jié)點(diǎn)的值的指針
    void *value;
} listNode;

typedef struct list {
    //頭節(jié)點(diǎn)指針
    listNode *head;
    //尾節(jié)點(diǎn)指針
    listNode *tail;
    //......
    //鏈表長(zhǎng)度
    unsigned long len;
} list;

list *listCreate(void)
{
    //為list結(jié)構(gòu)體分配內(nèi)存空間
    struct list *list;

    if ((list = zmalloc(sizeof(*list))) == NULL)
        return NULL;
    //頭尾指針初始化設(shè)置為空    
    list->head = list->tail = NULL;
    //鏈表長(zhǎng)度設(shè)置為0
    list->len = 0;
   //......
    return list;
}

list *listAddNodeHead(list *list, void *value)
{
    //初始化node節(jié)點(diǎn)內(nèi)存空間
    listNode *node;

    if ((node = zmalloc(sizeof(*node))) == NULL)
        return NULL;
    //value指針指向傳入的值    
    node->value = value;
    //如果鏈表長(zhǎng)度為0，則讓首尾節(jié)點(diǎn)指向這個(gè)node，然后node前驅(qū)和后繼節(jié)點(diǎn)為空
    if (list->len == 0) {
        list->head = list->tail = node;
        node->prev = node->next = NULL;
    } else {
        //節(jié)點(diǎn)的前驅(qū)指向空，后繼節(jié)點(diǎn)指向原有的頭節(jié)點(diǎn)，完成后再讓原有的頭節(jié)點(diǎn)作為新節(jié)點(diǎn)的后繼節(jié)點(diǎn)
        //最后head指針指向當(dāng)前node
        node->prev = NULL;
        node->next = list->head;
        list->head->prev = node;
        list->head = node;
    }
    //維護(hù)一下鏈表的長(zhǎng)度+1
    list->len++;
    return list;
}

list *listAddNodeTail(list *list, void *value)
{
    
    listNode *node;
    //分配node內(nèi)存空間
    if ((node = zmalloc(sizeof(*node))) == NULL)
        return NULL;
    //node的value指針指向value    
    node->value = value;
    //如果長(zhǎng)度為0，則首尾指針指向這個(gè)node
    if (list->len == 0) {
        list->head = list->tail = node;
        node->prev = node->next = NULL;
    } else {
        //新節(jié)點(diǎn)的前驅(qū)節(jié)點(diǎn)指向尾節(jié)點(diǎn)，然后讓原有尾節(jié)點(diǎn)指向新節(jié)點(diǎn)，最后讓tail指針指向新節(jié)點(diǎn)
        node->prev = list->tail;
        node->next = NULL;
        list->tail->next = node;
        list->tail = node;
    }
    //長(zhǎng)度信息維護(hù)一下
    list->len++;
    return list;
}

list *listInsertNode(list *list, listNode *old_node, void *value, int after) {
    listNode *node;
    //節(jié)點(diǎn)初始化并設(shè)置value
    if ((node = zmalloc(sizeof(*node))) == NULL)
        return NULL;
    node->value = value;
    //如果after為1則將新節(jié)點(diǎn)插入到old_node后面
    if (after) {
        //node前驅(qū)指向old_node，node指向old_node的后繼
        node->prev = old_node;
        node->next = old_node->next;
        //如果old_node是尾節(jié)點(diǎn)，則讓tail指向新插入的node
        if (list->tail == old_node) {
            list->tail = node;
        }
    } else {
        //將新節(jié)點(diǎn)插入到old_node前面
        node->next = old_node;
        node->prev = old_node->prev;
        //如果old_node是頭節(jié)點(diǎn)，則修改head指向，讓其指向新節(jié)點(diǎn)
        if (list->head == old_node) {
            list->head = node;
        }
    }
    //將node原有的前驅(qū)后繼節(jié)點(diǎn)指向當(dāng)前node維護(hù)的前驅(qū)和后繼節(jié)點(diǎn)
    if (node->prev != NULL) {
        node->prev->next = node;
    }
    if (node->next != NULL) {
        node->next->prev = node;
    }
    //維護(hù)一下長(zhǎng)度
    list->len++;
    return list;
}

listNode *listIndex(list *list, long index) {
    listNode *n;
    //如果小于0，說(shuō)明從后往前照
    if (index < 0) {
        //將負(fù)數(shù)轉(zhuǎn)為正數(shù)，例如傳入-2，也就找倒數(shù)第2個(gè)元素，轉(zhuǎn)為正為1，也就是往前1跳，返回這個(gè)node
        index = (-index)-1;
        n = list->tail;
        while(index-- && n) n = n->prev;
    } else {
        //說(shuō)明從前往后照，跳n跳即可得到對(duì)應(yīng)元素
        n = list->head;
        while(index-- && n) n = n->next;
    }
    return n;
}

void listDelNode(list *list, listNode *node)
{
    //如果node前驅(qū)有節(jié)點(diǎn)，則讓這個(gè)節(jié)點(diǎn)指向被刪除節(jié)點(diǎn)的后繼
    //反之說(shuō)明這個(gè)節(jié)點(diǎn)是頭節(jié)點(diǎn)，則讓head指向這個(gè)后繼節(jié)點(diǎn)
    if (node->prev)
        node->prev->next = node->next;
    else
        list->head = node->next;
    //如果這個(gè)節(jié)點(diǎn)有后繼節(jié)點(diǎn)，則讓這個(gè)后繼的prev指向被刪節(jié)點(diǎn)的前驅(qū)
    //反之說(shuō)明被刪的是尾節(jié)點(diǎn)，則讓tail指針指向被刪節(jié)點(diǎn)的后繼
    if (node->next)
        node->next->prev = node->prev;
    else
        list->tail = node->prev;
    //釋放被刪除節(jié)點(diǎn)的內(nèi)存空間，并減小鏈表長(zhǎng)度    
    if (list->free) list->free(node->value);
    zfree(node);
    list->len--;
}