leetcode之LRU实现

题目：

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before 

思路：

首先想到至少要维护两个数据结构，一个存key,value对本身（数据结构一），一个要记录访问记录排序，对第一个数据结构，由于需要快速查找，考虑用map存储，关于第二个数据结构，考虑两点，第一要顾及到排序，第二，要顾及到更新，因为有序是该数据结构的必然要求，而更新是该数据结构最常见的操作，基于这两点，考虑到用链表（数据结构二），但是链表有个问题，就是查找速度太慢，需要从头到尾扫描，这样的时间复杂度是肯定通不过leetcode OJ 的test case的，而在做更新的时候，是需要快速定位到某个节点的，所以要么把不用链表，换成其他数据结构，要么要再加一个数据结构以对链表进行记录，那么考虑用map吧（数据结构三）。有了这三个数据结构以后，考虑到数据结构二的更新操作涉及到节点移动，并需要记录最后一个节点（最近最少使用节点，这里把最近用过的节点都放在链表头部，最少使用的放在尾部，按最近访问顺序依次排序），设置一个类变量记录最后一个节点的地址，以减少搜索该节点的时间。

代码：

struct OrderNode{int key;OrderNode *next;OrderNode *prev;OrderNode(int x) : key(x), next(NULL), prev(NULL){};};class LRUCache{private:std::map<int, OrderNode*> orderfinder;  //find the previous node of the node in the order listOrderNode * orderlist;std::map<int, int> slot;int capacity;OrderNode * lastNode;public:LRUCache(int capacity) {this->capacity = capacity;orderlist = NULL;}int get(int key) {//not foundif (slot.find(key) == slot.end()) {return -1;}else  //found key{//record access for the node an update the access list;OrderNode* keyplace = orderfinder[key];//if the first nodeif (keyplace->prev == NULL){return slot[key];}keyplace->prev->next = keyplace->next;//if not lastNodeif (keyplace->next != NULL){keyplace->next->prev = keyplace->prev;}else{lastNode = lastNode->prev;lastNode->next = NULL;}keyplace->prev = NULL;keyplace->next = orderlist;orderlist->prev = keyplace;orderlist = keyplace;return slot[key];}}void set(int key, int value) {//initialization of empty orderlistif (orderlist == NULL){OrderNode *tmp = new OrderNode(key);orderlist = tmp;orderfinder.insert(std::pair<int, OrderNode*>(key, tmp));slot.insert(std::pair<int, int>(key, value));lastNode = tmp;}else if (slot.find(key) != slot.end()) //found the key already exist{//change the existing valueslot[key] = value;//update the orderlist by moving the node to firstOrderNode* keyplace = orderfinder[key];if (keyplace == orderlist)//already the in first place{return;}if (keyplace->next != NULL)// IF NOT THE LAST ELEMENT {keyplace->next->prev = keyplace->prev;}else{lastNode = keyplace->prev;}keyplace->prev->next = keyplace->next;keyplace->prev = NULL;keyplace->next = orderlist;orderlist->prev = keyplace;orderlist = keyplace;}else if ((int)slot.size()<capacity) //still has space to insert{//insert new nodeslot.insert(std::pair<int, int>(key, value));OrderNode* tmp = new OrderNode(key);tmp->next = orderlist;orderlist->prev = tmp;orderlist = tmp;orderfinder.insert(std::pair<int, OrderNode*>(key, tmp));}else //push the last node out of orderlist{//insert new nodeslot.insert(std::pair<int, int>(key, value));OrderNode* tmp = new OrderNode(key);if (capacity>1){tmp->next = orderlist;orderlist->prev = tmp;//update slot using delete element from mapslot.erase(slot.find(lastNode->key));OrderNode* pre = lastNode->prev;orderfinder.erase(orderfinder.find(lastNode->key));//update lastNode after pushing outlastNode = pre;lastNode->next = NULL;}else{tmp->next = NULL;slot.erase(slot.find(lastNode->key));orderfinder[lastNode->key] = tmp;OrderNode * p = orderlist;delete p;lastNode = tmp;}orderlist = tmp;orderfinder.insert(std::pair<int, OrderNode*>(key, tmp));}}};