LRU Cache

------QUESTION------

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 inserting a new item.

------SOLUTION------

class LRUCache{private:    struct ListNode{        ListNode * prev;        ListNode * next;        int key;        int value;        ListNode(int _key, int _value):key(_key), value(_value), prev(NULL), next(NULL){}      };       void change2TopPriotiy(ListNode* listNode){        if(head==listNode) return;        listNode->prev->next = listNode->next;        if(listNode->next) listNode->next->prev = listNode->prev;        else tail = tail->prev;        listNode->prev = NULL;        listNode->next = head;        head->prev = listNode;        head = head->prev;    }public:    LRUCache(int capacity) {        listNodeMap.clear();        this->capacity = capacity;        head = NULL;        tail = NULL;    }        int get(int key) {        if(listNodeMap.find(key)!=listNodeMap.end()){            change2TopPriotiy(listNodeMap[key]);            return listNodeMap[key]->value;        }        else return -1;    }        void set(int key, int value) {        if(listNodeMap.find(key)==listNodeMap.end()){ // ket not exist            if(listNodeMap.size()==capacity){ // reach the capacity                //erase least used data                listNodeMap.erase(tail->key);                ListNode * tmp = tail;                if(head == tail) {                    head = tail = NULL;                }                else{                    tail = tail->prev;                    tail->next = NULL;                }                delete tmp;            }            //insert new node            ListNode * newNode = new ListNode(key,value);            if(!head){ //first node in the list                head = tail = newNode;            }            else{                head->prev = newNode;                newNode->next = head;                head = head->prev;            }            listNodeMap[key]=newNode;        }        else{ //key already exists, change the value. eg: set(2,1),set(2,2),get(2) return 2            listNodeMap[key]->value = value;            change2TopPriotiy(listNodeMap[key]);        }    }private:    unordered_map< int,ListNode* > listNodeMap;    ListNode* head;    ListNode* tail;    int capacity;};