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跳表(skip list) 对标的是平衡树(AVL Tree),是一种 插入/删除/搜索 都是 O ( l o g n ) O(log n)O(logn) 的数据结构。它最大的优势是原理简单、容易实现、方便扩展、效率更高。因此在一些热门的项目里用来替代平衡树,如 redis, leveldb 等。
跳表顾名思义就是跳跃的表格,理解起来其实就是跳着插入或者搜索。具体什么意思呢,其实就像是二分搜索一样,每次都将数组分成两点分,先定位搜索或者插入的数据在哪一部分,就可以节约搜索的时间。跳表其实是一样的原理,即建立多层索引(多层链表)。如果每次都是以二等分来建立索引的话,即如下图所示:
但是上述结构是“静态”的,即我们先拥有了一个链表,再在之上建了多层的索引。但是在实际使用中,我们的链表是通过多次插入/删除形成的,换句话说是“动态”的。上述的结构要求上层相邻节点与对应下层节点间的个数比是 1:2,随意插入/删除一个节点,这个要求就被被破坏了。
因此跳表(skip list)表示,我们就不强制要求 1:2 了,一个节点要不要被索引,建几层的索引,都在节点插入时由抛硬币决定。当然,虽然索引的节点、索引的层数是随机的,为了保证搜索的效率,要大致保证每层的节点数目与上节的结构相当。下面是一个随机生成的跳表:
对于上述随机跳表而言,每次插入一个新结点的时候,该结点的索引层数是抛硬币决定的,即由随机算法决定的。当然为了防止运气太好,层数太高,我们一般会设置一个最大的层数 M a x L e v e l MaxLevelMaxLevel. 一般 M a x L e v e l = l o g 1 / p n MaxLevel=log_{1/p}nMaxLevel=log
1/p n。 p pp 为概率。首先创建一个结点的结构体,结构体里包含 一个键值对,key 用来建立索引,而 value 则用于存储真正的值;指向下一个结点的指针 next,以及标志每个结点索引层数的参数 level. next 指针是一个数组,用于存储结点在所有层数上的下一个结点。 例如, 上述图中结点6的next指针为{NIL, 25, 9, 7}.
创建class skipList, 包含头结点,尾结点,list 的最大层数,随机层数方法,以及一些列操作。 头结点和尾节点分别为整型的最小值和最大值,并在初始化时让所有层数上的头结点都指向尾节点。 操作包含插入,查找,删除。 SkipNode 为了可以适用于任何类型的value, 这里用了 templatetemplate//构造函数,初始化struct SkipNode{ int key; T value; vector next; SkipNode(int k, T v, int level);};
templateSkipNode ::SkipNode(int k, T v, int level) : key(k), value(v){ for (int i = 0; i < level; i++) { next.push_back(nullptr); }}
templateclass SkipList{public: //头结点 SkipNode * head; //列表最大层数 int maxLevel; //整型的最小值和最大值 const int minInt = numeric_limits ::min(); const int maxInt = numeric_limits ::max();public: //构造函数 SkipList(int maxLevel, T iniValue); //析构函数 ~SkipList(); //随机层数方法 int randomLevel(); //插入, 查找, 删除 SkipNode * insert(int k, T v); SkipNode * find(int k); SkipNode * deleteNode(int k); //打印 void printNode();private: //尾节点 SkipNode * tail; //找到当前列表或者node的最大层数 int nodeLevel(vector *> p);};//初始化template SkipList ::SkipList(int maxLevel, T iniValue) : maxLevel(maxLevel){ //初始化头结点和尾节点为整型最小值和最大值 head = new SkipNode (minInt, iniValue, maxLevel); tail = new SkipNode (maxInt, iniValue, maxLevel); //所有层数上的头结点指向尾节点 for (int i = 0; i < maxLevel; i++) { head->next[i] = tail; }}template SkipList ::~SkipList(){ delete head; delete tail;}
templateint SkipList ::randomLevel(){ int random_level = 1; int seed = time(NULL); static default_random_engine e(seed); static uniform_int_distribution u(0, 1); while (u(e) && random_level < maxLevel) { random_level++; } return random_level;}
templateint SkipList ::nodeLevel(vector *> next){ int node_level = 0; if (next[0]->key == maxInt) { return node_level; } for (int i = 0; i < next.size(); i++) { if (next[i] != nullptr && next[i]->key != maxInt) { node_level++; } else { break; } } return node_level;}
/*插入:1)首先用查找函数来判断该结点是否已经存在,如果存在,则更新该结点的值2)获取新节点的随机层数3)找到合适的插入位置4)插入,并调整每层前后node的指针*/templateSkipNode * SkipList ::insert(int k, T v){ int x_level = randomLevel(); SkipNode * new_node = nullptr; SkipNode * tmp = head; new_node = find(k); if (new_node) { new_node->value = v; cout << "\nThis node " << k << " has already existed. And its value has been updated to " << v << endl; return head; } cout << "key: " << k << ", randomLevel: " << x_level << endl; new_node = new SkipNode (k, v, x_level); for (int i = (x_level - 1); i > -1; i--) { while (tmp->next[i] != nullptr && tmp->next[i]->key < k) { tmp = tmp->next[i]; } new_node->next[i] = tmp->next[i]; tmp->next[i] = new_node; } return head;}/*查找:由于列表有序,首先找到小于该结点的最近的结点,如果下一个结点等于目标结点,则返回该节点。如果不是,则返回空*/template SkipNode * SkipList ::find(int x){ SkipNode * tmp = head; int current_level = nodeLevel(tmp->next); for (int i = (current_level - 1); i > -1; i--) { while (tmp->next[i] != nullptr && tmp->next[i]->key < x) { tmp = tmp->next[i]; } } tmp = tmp->next[0]; if (tmp->key == x) { cout << "\nThis key " << x << " has been found\n"; return tmp; } else { //cout << " \nThis key " << x << " doesn't exit\n"; return nullptr; }}/*删除:1) 用 find(x) 方法判断该结点是否存在. 如果不存在,则返回当前list, 并告知该结点不存在。2) 找到小于该结点的最近的结点。3) 更改该节点每层的前面的结点的指针。*/template SkipNode * SkipList ::deleteNode(int x){ SkipNode * node = find(x); if (!node) { cout << "\n This deleting node" << x << "doesn't exist" << endl; return head; } else { SkipNode * tmp = head; int x_level = node->next.size(); cout << "\nThe deleting node " << x << "'s level is " << x_level << endl; for (int i = (x_level - 1); i > -1; i--) { while (tmp->next[i] != nullptr && tmp->next[i]->key < x) { tmp = tmp->next[i]; } tmp->next[i] = tmp->next[i]->next[i]; cout << "This node " << x << " has been deleted from level " << i << endl; } return head; }}
// 分层打印templatevoid SkipList ::printNode(){ for (int i = 0; i < maxLevel; i++) { SkipNode * tmp = head; int lineLen = 1; if (tmp->next[i]->key != maxInt) { cout << "\n"; cout << "This is level " << i << ":" << endl; cout << "{"; while (tmp->next[i] != nullptr && tmp->next[i]->key != maxInt) { cout << "(" << "Key: " << tmp->next[i]->key << ", "; cout << "Value: " << tmp->next[i]->value << ")"; tmp = tmp->next[i]; if (tmp->next[i] != nullptr && tmp->next[i]->key != maxInt) { cout << ", "; } if (lineLen++ % 5 == 0) cout << "\n"; } cout << "}" << "\n"; } }}
int main(){ int maxLevel = 6; SkipList l(maxLevel, 0); for (size_t i = 0; i < 50; i++) { l.insert(i, i); } l.printNode();
This is level 0:{(Key: 0, Value: 0), (Key: 1, Value: 1), (Key: 2, Value: 2), (Key: 3, Value: 3), (Key: 4, Value: 4),(Key: 5, Value: 5), (Key: 6, Value: 6), (Key: 7, Value: 7), (Key: 8, Value: 8), (Key: 9, Value: 9),(Key: 10, Value: 10), (Key: 11, Value: 11), (Key: 12, Value: 12), (Key: 13, Value: 13), (Key: 14, Value: 14),(Key: 15, Value: 15), (Key: 16, Value: 16), (Key: 17, Value: 17), (Key: 18, Value: 18), (Key: 19, Value: 19),(Key: 20, Value: 20), (Key: 21, Value: 21), (Key: 22, Value: 22), (Key: 23, Value: 23), (Key: 24, Value: 24),(Key: 25, Value: 25), (Key: 26, Value: 26), (Key: 27, Value: 27), (Key: 28, Value: 28), (Key: 29, Value: 29),(Key: 30, Value: 30), (Key: 31, Value: 31), (Key: 32, Value: 32), (Key: 33, Value: 33), (Key: 34, Value: 34),(Key: 35, Value: 35), (Key: 36, Value: 36), (Key: 37, Value: 37), (Key: 38, Value: 38), (Key: 39, Value: 39),(Key: 40, Value: 40), (Key: 41, Value: 41), (Key: 42, Value: 42), (Key: 43, Value: 43), (Key: 44, Value: 44),(Key: 45, Value: 45), (Key: 46, Value: 46), (Key: 47, Value: 47), (Key: 48, Value: 48), (Key: 49, Value: 49)}This is level 1:{(Key: 0, Value: 0), (Key: 4, Value: 4), (Key: 6, Value: 6), (Key: 10, Value: 10), (Key: 14, Value: 14),(Key: 15, Value: 15), (Key: 17, Value: 17), (Key: 21, Value: 21), (Key: 22, Value: 22), (Key: 23, Value: 23),(Key: 28, Value: 28), (Key: 30, Value: 30), (Key: 34, Value: 34), (Key: 35, Value: 35), (Key: 36, Value: 36),(Key: 38, Value: 38), (Key: 39, Value: 39), (Key: 42, Value: 42), (Key: 46, Value: 46), (Key: 48, Value: 48),(Key: 49, Value: 49)}This is level 2:{(Key: 4, Value: 4), (Key: 21, Value: 21), (Key: 23, Value: 23), (Key: 34, Value: 34), (Key: 35, Value: 35),(Key: 39, Value: 39), (Key: 42, Value: 42), (Key: 46, Value: 46)}This is level 3:{(Key: 4, Value: 4), (Key: 46, Value: 46)}This is level 4:{(Key: 46, Value: 46)}———————————————— 版权声明:本文为CSDN博主「啦啦啦啦啦~~」的原创文章,遵循CC 4.0 BY-SA版权协议,转载请附上原文出处链接及本声明。 原文链接:https://blog.csdn.net/weixin_44387066/article/details/90766034