SkipList的实现

当做笔记记录下来:)

相比于bTree，skiplist占用更多的空间。

而在并发的环境下skiplist会比bTree效率更高。

(bTree插入的时候需要rebalance，需要lock比较多的结点，而skiplist则只需要lock跟node相关的几个结点)

SkipList.h

 #define MAX_LEVEL 16

 typedef int KeyType;
 typedef int ValueType;

 typedef struct nodeStructure* Node;
 struct nodeStructure
 {
     KeyType key;
     ValueType value;
     Node forward[];
 };

 class SkipList
 {
 public:
     SkipList();
     Node createNode(int level, const KeyType& key, const ValueType& value);
     void createList();
     bool insert(const KeyType& key, const ValueType& value);
     bool erase(const KeyType& key, ValueType& value);
     bool search(const KeyType& key, ValueType& value);
 private:
     int randomLevel();
 private:
     int _level;
     Node _header;
 };

SkipList.cpp

 #include "SkipList.h"
 #include <iostream>

 SkipList::SkipList()
 {
     _level = ;
     _header = createNode(MAX_LEVEL, , );
     Node tail = createNode(, 0x7fffffff, );
     for (int i = ; i < MAX_LEVEL; ++i)
         _header->forward[i] = tail;

 }

 Node SkipList::createNode(int level, const KeyType& key, const ValueType& value)
 {
     Node node = (Node)malloc(
         sizeof(nodeStructure) + sizeof(Node) * level);
     node->key = key;
     node->value = value;
     return node;
 }

 int SkipList::randomLevel()
 {
     int k = ;
     // 50% k++
     while (rand() % )
         k++;
     return (k < MAX_LEVEL) ? k : MAX_LEVEL - ;
 }

 bool SkipList::insert(const KeyType& key, const ValueType& value)
 {
     Node update[MAX_LEVEL];
     Node node = _header;
     // search
     for (int i = _level; i >= ; --i)
     {
         while (node->forward[i]->key < key)
             node = node->forward[i];
         // record previous node
         update[i] = node;
     }

     // same key
     if (node->forward[]->key == key)
         return false;

     int level = randomLevel();
     // update level
     if (level > _level)
     {
         for (int i = _level + ; i <= level; ++i)
             update[i] = _header;
         _level = level;
     }

     // update pointer
     Node insNode = createNode(level, key, value);
     for (int i = level; i >= ; --i)
     {
         node = update[i];
         insNode->forward[i] = node->forward[i];
         node->forward[i] = insNode;
     }
     return true;
 }

 // similar to insert
 bool SkipList::erase(const KeyType& key, ValueType& value)
 {
     Node update[MAX_LEVEL];
     Node node = _header;
     for (int i = _level; i >= ; --i)
     {
         while (node->forward[i]->key < key)
             node = node->forward[i];
         update[i] = node;
     }

     node = node->forward[];
     if (node->key != key)
         return false;

     for (int i = ; i < _level; ++i)
     {
         if (update[i]->forward[i] != node)
             break;
         update[i]->forward[i] = node->forward[i];
     }
     free(node);
     return true;
 }

 bool SkipList::search(const KeyType& key, ValueType& value)
 {
     Node node = _header;
     for (int i = _level; i >= ; --i)
         // find the last node->key < key
         while (node->forward[i]->key < key)
             node = node->forward[i];
     // node->key >= key
     node = node->forward[];
     if (node->key == key)
     {
         value = node->value;
         return true;
     }
     return false;
 }