redis中并没有专门给跳跃表两个文件。在5.0.2的版本中，结构体的声明与定义、接口的声明在server.h中，接口的定义在t_zset.c中，所有开头为zsl的函数。
一、数据结构
单个节点：
1 /**
2 * ZSETs use a specialized version of Skiplists
3 * ZSET 使用专门版本的 Skiplist（跳跃表），跳跃表节点
4 */
5 typedef struct zskiplistNode {
6 //key，唯一
7 sds ele;
8 //分值，可重复
9 double score;
10 //后退指针
11 struct zskiplistNode *backward;
12 //层
13 struct zskiplistLevel {
14 //前进指针
15 struct zskiplistNode *forward;
16 //到本层下一节点的跨度，用于计算rank
17 unsigned long span;
18 } level[];
19 } zskiplistNode;
zskiplistNode定义了跳跃表中每个节点的数据结构，它是一个变长结构体。
1 /* 2 +------------------------+ 3 |sds ele | /+-----------------------------+ 4 +------------------------+ / |struct zskiplistNode *forward| 5 |double score | / +-----------------------------+ 6 +------------------------+ / |unsigned long span | 7 |zskiplistNode * backward| / +-----------------------------+ 8 +------------------------+/ . . 9 |zskiplistLevel level[] | . . 10 +------------------------+\ . . 11 \ +-----------------------------+ 12 \ |struct zskiplistNode *forward| 13 \ +-----------------------------+ 14 \ |unsigned long span | 15 \+-----------------------------+ 16 */
下面将用以下结构表示
1 /* 2 +--------+ 3 |level[1]| 4 |1(span) | 5 +--------+ 6 |level[0]| 7 |1(span) | 8 +--------+ 9 |backward| 10 +--------+ 11 |score | 12 +--------+ 13 |ele | 14 +--------+ 15 */
例如
1 /* 2 +--------+ +--------+ +--------+ 3 |level[1]|--------------->|level[1]|--------------->|level[1]| 4 |2 | |2 | |0 | 5 +--------+ +--------+ +--------+ +--------+ +--------+ 6 |level[0]|-->|level[0]|-->|level[0]|-->|level[0]|-->|level[0]| 7 |1 | |1 | |1 | |1 | |0 | 8 +--------+ +--------+ +--------+ +--------+ +--------+ 9 |backward|<--|backward|<--|backward|<--|backward|<--|backward| 10 +--------+ +--------+ +--------+ +--------+ +--------+ 11 |1 | |2 | |3 | |4 | |5 | 12 +--------+ +--------+ +--------+ +--------+ +--------+ 13 |a | |b | |c | |d | |e | 14 +--------+ +--------+ +--------+ +--------+ +--------+ 15 */
跳跃表定义
1 /**
2 * 跳跃表
3 */
4 typedef struct zskiplist {
5 //头/尾节点
6 struct zskiplistNode *header, *tail;
7 //总长度
8 unsigned long length;
9 //总层数
10 int level;
11 } zskiplist;
因其头节点固定为空节点，固整体结构：
1 /* 2 +--------+ +--------+ +--------+ 3 |level[1]|--------------->|level[1]|--------------->|level[1]| 4 |2 | |2 | |0 | 5 +--------+ +--------+ +--------+ +--------+ +--------+ 6 |level[0]|-->|level[0]|-->|level[0]|-->|level[0]|-->|level[0]| 7 |1 | |1 | |1 | |1 | |0 | 8 +--------+ +--------+ +--------+ +--------+ +--------+ 9 |backward|<--|backward|<--|backward|<--|backward|<--|backward| 10 +--------+ +--------+ +--------+ +--------+ +--------+ 11 |0 | |2 | |3 | |4 | |5 | 12 +--------+ +--------+ +--------+ +--------+ +--------+ 13 |NULL | |b | |c | |d | |e | 14 +-->+--------+ +--------+ +--------+ +--------+ +--------+<--+ 15 | | 16 | +--------+ | 17 +---|header | | 18 +--------+ | 19 |tail |-------------------------------------------------------+ 20 +--------+ 21 |length=4| 22 +--------+ 23 |level=2 | 24 +--------+ 25 */
每个level层都是一条单身链表，其中level[0]中包含所有元素。
二、创建
根据指定的level，创建一个跳表节点：
1 /**
2 * Create a skiplist node with the specified number of levels.
3 * The SDS string 'ele' is referenced by the node after the call.
4 * 创建一个跳跃表节点
5 */
6 zskiplistNode *zslCreateNode(int level, double score, sds ele) {
7 //分配内存
8 zskiplistNode *zn =
9 zmalloc(sizeof(*zn)+level*sizeof(struct zskiplistLevel));
10 //设置分数
11 zn->score = score;
12 //key
13 zn->ele = ele;
14 //返回创建的结构
15 return zn;
16 }
创建一个跳表
1 /**
2 * Create a new skiplist.
3 * 创建一个跳跃表
4 */
5 zskiplist *zslCreate(void) {
6 int j;
7 zskiplist *zsl;
8
9 //分配内存
10 zsl = zmalloc(sizeof(*zsl));
11 //设置初始层为1
12 zsl->level = 1;
13 //跳跃表长度为0
14 zsl->length = 0;
15 //创建并初始化头节点，跳跃表最大层级为64，头节点的key为NULL，分数为0
16 zsl->header = zslCreateNode(ZSKIPLIST_MAXLEVEL,0,NULL);
17
18 //初始化头节点中level元素，前进指针为NULL，到本层下一节点的跨度span为0，用于计算rank
19 for (j = 0; j < ZSKIPLIST_MAXLEVEL; j++) {
20 zsl->header->level[j].forward = NULL;
21 zsl->header->level[j].span = 0;
22 }
23 zsl->header->backward = NULL;
24 zsl->tail = NULL;
25 //返回创建的跳跃表
26 return zsl;
27 }
redis中定义的最大层数为64层。且在刚创建时，会生成一个空的头节点，这样就可以不用再考虑节点数从0至1或者从1至0时要处理的各种特殊情况。
刚创完的跳表结构（在后面的分析中以4做为最大层数）
1 /* 2 +--------+ 3 |level[3]|-->NULL 4 |0 | 5 +--------+ 6 |level[2]|-->NULL 7 |0 | 8 +--------+ 9 |level[1]|-->NULL 10 |0 | 11 +--------+ 12 |level[0]|-->NULL 13 |0 | 14 +--------+ 15 NULL<-|backward| 16 +--------+ 17 |0 | 18 +--------+ 19 |NULL | 20 +-->+--------+ 21 | 22 | +--------+ 23 +---|header | 24 +--------+ 25 |tail |-->NULL 26 +--------+ 27 |length=0| 28 +--------+ 29 |level=1 | 30 +--------+ 31 */
三、插入节点
1 /**
2 * Returns a random level for the new skiplist node we are going to create.
3 * The return value of this function is between 1 and ZSKIPLIST_MAXLEVEL
4 * (both inclusive), with a powerlaw-alike distribution where higher
5 * levels are less likely to be returned.
6 * 跳跃表中用来决定跨越几层的抛硬币算法，正面概率设置为0.25
7 */
8 int zslRandomLevel(void) {
9 int level = 1;
10 /**
11 * #define ZSKIPLIST_P 0.25
12 */
13 while ((random()&0xFFFF) < (ZSKIPLIST_P * 0xFFFF))
14 level += 1;
15 return (level<ZSKIPLIST_MAXLEVEL) ? level : ZSKIPLIST_MAXLEVEL;
16 }
需要注意的是redis中使用的决定新插入节点层数据的方法是抛硬币法，且“硬币”只有25%的几率是正面。
插入方法：
1 /**
2 * Insert a new node in the skiplist. Assumes the element does not already
3 * exist (up to the caller to enforce that). The skiplist takes ownership
4 * of the passed SDS string 'ele'.
5 * 在跳跃表中插入一个新节点，假设该元素尚不存在（由调用者强制保证），跳过列表获得传递的 SDS 字符串“ele”的所有权。
6 * ele就相当于key
7 */
8 zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {
9 //update数组，用于存储查找路径
10 zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
11 //rank数组，用于存储每层路径节点的排名
12 unsigned int rank[ZSKIPLIST_MAXLEVEL];
13 int i, level;
14
15 serverAssert(!isnan(score));
16 x = zsl->header;
17
18 //先查找插入位置，从最上面的一层开始查找
19 for (i = zsl->level-1; i >= 0; i--) {
20 /* store rank that is crossed to reach the insert position */
21 rank[i] = i == (zsl->level-1) ? 0 : rank[i+1];
22 /**
23 * 当当前层级的下一个节点不为空并且（下一个节点的分数小于插入节点的分数或者
24 * 二者分数相等但是插入的ele小于下一个节点的ele）就遍历当前层的下一个节点，否者记录查找路径，
25 * 并从当前位置开始遍历下一层，指导遍历到第0层
26 */
27 while (x->level[i].forward &&
28 (x->level[i].forward->score < score ||
29 (x->level[i].forward->score == score &&
30 sdscmp(x->level[i].forward->ele,ele) < 0)))
31 {
32 rank[i] += x->level[i].span;
33 x = x->level[i].forward;
34 }
35 update[i] = x;
36 }
37 /**
38 * we assume the element is not already inside, since we allow duplicated
39 * scores, reinserting the same element should never happen since the
40 * caller of zslInsert() should test in the hash table if the element is
41 * already inside or not.
42 * 利用抛硬币算法的插入元素跨越的层数
43 */
44 level = zslRandomLevel();
45 if (level > zsl->level) {
46 //如果跨越的层数大于当前层，更新查找路径
47 for (i = zsl->level; i < level; i++) {
48 rank[i] = 0;
49 update[i] = zsl->header;
50 update[i]->level[i].span = zsl->length;
51 }
52 //更新当前跳跃表中的实际层数
53 zsl->level = level;
54 }
55
56 //创建插入的跳跃表节点
57 x = zslCreateNode(level,score,ele);
58 //在跨越的每一层插入创建的节点
59 for (i = 0; i < level; i++) {
60 //插入当前层的链表当中
61 x->level[i].forward = update[i]->level[i].forward;
62 update[i]->level[i].forward = x;
63
64 /**
65 * update span covered by update[i] as x is inserted here
66 * 更新插入位置相邻两个节点的跨度也就是span
67 */
68 x->level[i].span = update[i]->level[i].span - (rank[0] - rank[i]);
69 update[i]->level[i].span = (rank[0] - rank[i]) + 1;
70 }
71
72 /**
73 * increment span for untouched levels
74 * 当插入元素的跨度小于zsl->level，更新level到zsl->level每一层查找路径中元素的跨度，因为当前层没有元素插入
75 * 所以需要跨度增加了1
76 */
77 for (i = level; i < zsl->level; i++) {
78 update[i]->level[i].span++;
79 }
80
81 /**
82 * 设置节点的后退指针
83 */
84 x->backward = (update[0] == zsl->header) ? NULL : update[0];
85 if (x->level[0].forward)
86 x->level[0].forward->backward = x;
87 else
88 zsl->tail = x;
89 //更新第0层链表的长度
90 zsl->length++;
91 return x;
92 }
从注释可知，redis的跳表允许同score的情况发生，但是不允许同ele，且是由调用者在外部保证。若插入顺序为e，b，c，d，则插入e时：
step1、定义update数组与rank数组。
1 /* 2 update rank 3 +--------+ +--------+ 4 |NULL | |0 | 5 +--------+ +--------+ 6 |NULL | |0 | 7 +--------+ +--------+ 8 |NULL | |0 | 9 +--------+ +--------+ 10 |NULL | |0 | 11 +--------+ +--------+ 12 */
实际在linux环境运行时，不会默认初始化，应该是一堆脏数据，此处是为了方便处理结构
step2、查找位置后
1 /* 2 update rank 3 +--------+ +--------+ 4 |NULL | |0 | 5 +--------+ +--------+ 6 |NULL | |0 | 7 +--------+ +--------+ 8 |NULL | |0 | 9 +--------+ +--------+ 10 |header | |0 | 11 +--------+ +--------+ 12 */
step3、e的level为2，比跳表的大，故要补齐update与rank数组
1 /* 2 update rank 3 +--------+ +--------+ 4 |NULL | |0 | 5 +--------+ +--------+ 6 |NULL | |0 | 7 +--------+ +--------+ 8 |header | |0 | 9 +--------+ +--------+ 10 |header | |0 | 11 +--------+ +--------+ 12 */
step4、插入节点，与单身链表插入相同，将新节点e各层，插入到update数组中记录的各层节点之后，并使用rank数组，计算跨度
1 /* 2 +--------+ 3 |level[3]|-->NULL 4 |0 | 5 +--------+ 6 |level[2]|-->NULL 7 |0 | 8 +--------+ +--------+ 9 |level[1]|-->|level[1]|-->NULL 10 |1 | |0 | 11 +--------+ +--------+ 12 |level[0]|-->|level[0]|-->NULL 13 |1 | |0 | 14 +--------+ +--------+ 15 NULL<-|backward| |backward| 16 +--------+ +--------+ 17 |0 | |5 | 18 +--------+ +--------+ 19 |NULL | |e | 20 +-->+--------+ +--------+ 21 | 22 | +--------+ 23 +---|header | 24 +--------+ 25 |tail | 26 +--------+ 27 |length=0| 28 +--------+ 29 |level=1 | 30 +--------+ 31 */
step5、处理新插入节点的backward指针，与跳表的tail指针
1 /* 2 +--------+ 3 |level[3]|-->NULL 4 |0 | 5 +--------+ 6 |level[2]|-->NULL 7 |0 | 8 +--------+ +--------+ 9 |level[1]|-->|level[1]|-->NULL 10 |1 | |0 | 11 +--------+ +--------+ 12 |level[0]|-->|level[0]|-->NULL 13 |1 | |0 | 14 +--------+ +--------+ 15 NULL<-|backward| |backward| 16 +--------+ +--------+ 17 |0 | |5 | 18 +--------+ +--------+ 19 |NULL | |e | 20 +-->+--------+ +--------+<--+ 21 | | 22 | +--------+ | 23 +---|header | | 24 +--------+ | 25 |tail |----------------+ 26 +--------+ 27 |length=1| 28 +--------+ 29 |level=2 | 30 +--------+ 31 32 */
此时插入b：
找到位置后的update与rank数组
1 /* 2 update rank 3 +--------+ +--------+ 4 |NULL | |0 | 5 +--------+ +--------+ 6 |NULL | |0 | 7 +--------+ +--------+ 8 |header | |0 | 9 +--------+ +--------+ 10 |header | |0 | 11 +--------+ +--------+ 12 */
插入b节点后：
1 /* 2 +--------+ 3 |level[3]|-->NULL 4 |0 | 5 +--------+ 6 |level[2]|-->NULL 7 |0 | 8 +--------+ +--------+ 9 |level[1]|--------------->|level[1]|-->NULL 10 |2 | |0 | 11 +--------+ +--------+ +--------+ 12 |level[0]|-->|level[0]|-->|level[0]|-->NULL 13 |1 | |1 | |0 | 14 +--------+ +--------+ +--------+ 15 NULL<-|backward| |backward|<--|backward| 16 +--------+ +--------+ +--------+ 17 |0 | |2 | |5 | 18 +--------+ +--------+ +--------+ 19 |NULL | |b | |e | 20 +-->+--------+ +--------+ +--------+<--+ 21 | | 22 | +--------+ | 23 +---|header | | 24 +--------+ | 25 |tail |-----------------------------+ 26 +--------+ 27 |length=2| 28 +--------+ 29 |level=2 | 30 +--------+ 31 */
需要注意的是，update数组idx = 1的节点并没有新的插入操作，span要自增，表示本层跨度增加了1。
插入c时的update与rank数组：
1 /* 2 update rank 3 +--------+ +--------+ 4 |NULL | |0 | 5 +--------+ +--------+ 6 |NULL | |0 | 7 +--------+ +--------+ 8 |header | |0 | 9 +--------+ +--------+ 10 |b | |1 | 11 +--------+ +--------+ 12 */
插入c后
1 /* 2 +--------+ 3 |level[3]|-->NULL 4 |0 | 5 +--------+ 6 |level[2]|-->NULL 7 |0 | 8 +--------+ +--------+ +--------+ 9 |level[1]|--------------->|level[1]|-->|level[1]|-->NULL 10 |2 | |1 | |0 | 11 +--------+ +--------+ +--------+ +--------+ 12 |level[0]|-->|level[0]|-->|level[0]|-->|level[0]|-->NULL 13 |1 | |1 | |1 | |0 | 14 +--------+ +--------+ +--------+ +--------+ 15 NULL<-|backward| |backward|<--|backward|<--|backward| 16 +--------+ +--------+ +--------+ +--------+ 17 |0 | |2 | |3 | |5 | 18 +--------+ +--------+ +--------+ +--------+ 19 |NULL | |b | |c | |e | 20 +-->+--------+ +--------+ +--------+ +--------+<--+ 21 | | 22 | +--------+ | 23 +---|header | | 24 +--------+ | 25 |tail |------------------------------------------+ 26 +--------+ 27 |length=3| 28 +--------+ 29 |level=2 | 30 +--------+ 31 /*
最后插入d
update与rank数组
1 /* 2 update rank 3 +--------+ +--------+ 4 |NULL | |0 | 5 +--------+ +--------+ 6 |NULL | |0 | 7 +--------+ +--------+ 8 |c | |2 | 9 +--------+ +--------+ 10 |c | |2 | 11 +--------+ +--------+ 12 */
插入d
1 /* 2 +--------+ 3 |level[3]|-->NULL 4 |0 | 5 +--------+ 6 |level[2]|-->NULL 7 |0 | 8 +--------+ +--------+ +--------+ 9 |level[1]|--------------->|level[1]|--------------->|level[1]|-->NULL 10 |2 | |2 | |0 | 11 +--------+ +--------+ +--------+ +--------+ +--------+ 12 |level[0]|-->|level[0]|-->|level[0]|-->|level[0]|-->|level[0]|-->NULL 13 |1 | |1 | |1 | |1 | |0 | 14 +--------+ +--------+ +--------+ +--------+ +--------+ 15 NULL<-|backward| |backward|<--|backward|<--|backward|<--|backward| 16 +--------+ +--------+ +--------+ +--------+ +--------+ 17 |0 | |2 | |3 | |4 | |5 | 18 +--------+ +--------+ +--------+ +--------+ +--------+ 19 |NULL | |b | |c | |d | |e | 20 +-->+--------+ +--------+ +--------+ +--------+ +--------+<--+ 21 | | 22 | +--------+ | 23 +---|header | | 24 +--------+ | 25 |tail |-------------------------------------------------------+ 26 +--------+ 27 |length=4| 28 +--------+ 29 |level=2 | 30 +--------+ 31 /*
如果此时要新插入节点a，score为4.5，则update与rank数组分别为
1 /* 2 update rank 3 +--------+ +--------+ 4 |NULL | |0 | 5 +--------+ +--------+ 6 |NULL | |0 | 7 +--------+ +--------+ 8 |c | |2 | 9 +--------+ +--------+ 10 |d | |3 | 11 +--------+ +--------+ 12 */
四、删除节点
在已经查找到位置，与已知update数组时的删除方法
1 /**
2 * Internal function used by zslDelete, zslDeleteByScore and zslDeleteByRank
3 * 被zslDelete, zslDeleteByScore and zslDeleteByRank调用的内部函数，在删除本节点之后，对应路径相应得做处理。
4 *
5 * 在level[0]层，使用从单向链表中删除节点的操作，把删除节点移出，
6 * 再给高于level[0]的update数组中所有成员的span自减，节点少了，跨度要跟着降低。
7 */
8 void zslDeleteNode(zskiplist *zsl, zskiplistNode *x, zskiplistNode **update) {
9 int i;
10 for (i = 0; i < zsl->level; i++) {
11 if (update[i]->level[i].forward == x) {
12 //更新节点的跨度
13 update[i]->level[i].span += x->level[i].span - 1;
14 //更新节点的前进指针
15 update[i]->level[i].forward = x->level[i].forward;
16 } else {
17 update[i]->level[i].span -= 1;
18 }
19 }
20 //设置后退指针
21 if (x->level[0].forward) {
22 x->level[0].forward->backward = x->backward;
23 } else {
24 zsl->tail = x->backward;
25 }
26 //修正跳跃表的实际层数
27 while(zsl->level > 1 && zsl->header->level[zsl->level-1].forward == NULL)
28 zsl->level--;
29 //更新跳跃表的长度
30 zsl->length--;
31 }
删除本节点之后，对应路径相应得做处理。
从跳表中删除指定节点的操作
1 /* Delete an element with matching score/element from the skiplist.
2 * The function returns 1 if the node was found and deleted, otherwise
3 * 0 is returned.
4 * 删除跳跃表中匹配到的分数和元素，如果该节点被找到且删除返回1，否则返回0
5 *
6 * If 'node' is NULL the deleted node is freed by zslFreeNode(), otherwise
7 * it is not freed (but just unlinked) and *node is set to the node pointer,
8 * so that it is possible for the caller to reuse the node (including the
9 * referenced SDS string at node->ele).
10 * 如果node参数为NULL，删除的节点如果被找到，就需要在该函数中释放该节点所占有的内存空间，否者，将找到的该节点
11 * 赋值给*node，从而使得调用者在使用完该节点的时候再调用相应的函数zslFreeNode将其释放
12 */
13 int zslDelete(zskiplist *zsl, double score, sds ele, zskiplistNode **node) {
14 zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
15 int i;
16
17 //查找节点并记录路径
18 x = zsl->header;
19 for (i = zsl->level-1; i >= 0; i--) {
20 while (x->level[i].forward &&
21 (x->level[i].forward->score < score ||
22 (x->level[i].forward->score == score &&
23 sdscmp(x->level[i].forward->ele,ele) < 0)))
24 {
25 x = x->level[i].forward;
26 }
27 update[i] = x;
28 }
29 /**
30 * We may have multiple elements with the same score, what we need
31 * is to find the element with both the right score and object.
32 */
33 x = x->level[0].forward;
34 if (x && score == x->score && sdscmp(x->ele,ele) == 0) {
35 //根据准备删除的节点和查找路径对跳跃表做相应的处理
36 zslDeleteNode(zsl, x, update);
37 if (!node)//如果node不为NULL，就直接释放该节点的内存
38 zslFreeNode(x);
39 else
40 *node = x;//利用传出参数被调用者使用
41 return 1;
42 }
43 return 0; /* not found */
44 }
如以下跳表
1 /* 2 +--------+ 3 |level[3]|-->NULL 4 |0 | 5 +--------+ 6 |level[2]|-->NULL 7 |0 | 8 +--------+ +--------+ +--------+ 9 |level[1]|--------------->|level[1]|--------------->|level[1]|-->NULL 10 |2 | |2 | |0 | 11 +--------+ +--------+ +--------+ +--------+ +--------+ 12 |level[0]|-->|level[0]|-->|level[0]|-->|level[0]|-->|level[0]|-->NULL 13 |1 | |1 | |1 | |1 | |0 | 14 +--------+ +--------+ +--------+ +--------+ +--------+ 15 NULL<-|backward| |backward|<--|backward|<--|backward|<--|backward| 16 +--------+ +--------+ +--------+ +--------+ +--------+ 17 |0 | |2 | |3 | |4 | |5 | 18 +--------+ +--------+ +--------+ +--------+ +--------+ 19 |NULL | |b | |c | |d | |e | 20 +-->+--------+ +--------+ +--------+ +--------+ +--------+<--+ 21 | | 22 | +--------+ | 23 +---|header | | 24 +--------+ | 25 |tail |-------------------------------------------------------+ 26 +--------+ 27 |length=4| 28 +--------+ 29 |level=2 | 30 +--------+ 31 /*
要删除节点d，生成的update数组为
1 /* 2 update 3 +--------+ 4 |NULL | 5 +--------+ 6 |NULL | 7 +--------+ 8 |c | 9 +--------+ 10 |c | 11 +--------+ 12 */
由于d的level为1，故在level[0]层，使用从单向链表中删除节点的操作，把d移出，再给高于level[0]的update数组中所有成员的span自减，节点少了，跨度要跟着降低。
删除d之后的跳表
1 /* 2 +--------+ 3 |level[3]|-->NULL 4 |0 | 5 +--------+ 6 |level[2]|-->NULL 7 |0 | 8 +--------+ +--------+ +--------+ 9 |level[1]|--------------->|level[1]|-->|level[1]|-->NULL 10 |2 | |1 | |0 | 11 +--------+ +--------+ +--------+ +--------+ 12 |level[0]|-->|level[0]|-->|level[0]|-->|level[0]|-->NULL 13 |1 | |1 | |1 | |0 | 14 +--------+ +--------+ +--------+ +--------+ 15 NULL<-|backward| |backward|<--|backward|<--|backward| 16 +--------+ +--------+ +--------+ +--------+ 17 |0 | |2 | |3 | |5 | 18 +--------+ +--------+ +--------+ +--------+ 19 |NULL | |b | |c | |e | 20 +-->+--------+ +--------+ +--------+ +--------+<--+ 21 | | 22 | +--------+ | 23 +---|header | | 24 +--------+ | 25 |tail |------------------------------------------+ 26 +--------+ 27 |length=3| 28 +--------+ 29 |level=2 | 30 +--------+ 31 /*
五、销毁
1 /**
2 * Free the specified skiplist node. The referenced SDS string representation
3 * of the element is freed too, unless node->ele is set to NULL before calling
4 * this function.
5 * 释放指定跳跃节点的内存，需要释放成员变量ele的内存，然后再释放node，除非ele被设置为NULL
6 */
7 void zslFreeNode(zskiplistNode *node) {
8 sdsfree(node->ele);
9 zfree(node);
10 }
11
12 /* Free a whole skiplist. 释放整个跳跃表*/
13 void zslFree(zskiplist *zsl) {
14 zskiplistNode *node = zsl->header->level[0].forward, *next;
15
16 zfree(zsl->header);
17 while(node) {
18 next = node->level[0].forward;
19 zslFreeNode(node);
20 node = next;
21 }
22 zfree(zsl);
23 }
销毁操作本身只是在level[0]层遍历所有节点，依次销毁。
参考文章：
https://www.cnblogs.com/chinxi/p/12259603.html