算法描述：

Design and implement a data structure for . It should support the following operations: get and put.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.

put(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.

Follow up:

Could you do both operations in O(1) time complexity?

Example:

LRUCache cache = new LRUCache( 2 /* capacity */ );cache.put(1, 1);cache.put(2, 2);cache.get(1);       // returns 1cache.put(3, 3);    // evicts key 2cache.get(2);       // returns -1 (not found)cache.put(4, 4);    // evicts key 1cache.get(1);       // returns -1 (not found)cache.get(3);       // returns 3cache.get(4);       // returns 4

解题思路：时间复杂度为O(1)，所以为了便于查找必须采用hash尽力映射。而为了便于插入删除，采用双端链表存储数据。需要注意的是容量不够需要删除尾部元素。此外，查到map中有key存在，需要替换value值，否则有些case过不去。

struct DoubleListNode{    int key;    int val;    DoubleListNode* prev;    DoubleListNode* next;    DoubleListNode(int x, int y): key(x), val(y), prev(nullptr), next(nullptr){}};class LRUCache {public:    LRUCache(int capacity) : _capacity(capacity) {        head->next=tail;        tail->prev=head;    }        int get(int key) {        if(dMap.find(key)==dMap.end()){            return -1;        }else{            dMap[key]->prev->next = dMap[key]->next;            dMap[key]->next->prev = dMap[key]->prev;            dMap[key]->next = head->next;            head->next->prev = dMap[key];            head->next = dMap[key];            dMap[key]->prev = head;            return dMap[key]->val;        }    }        void put(int key, int value) {        if(dMap.find(key) == dMap.end()){            if(_capacity==0){                DoubleListNode* temp = tail->prev;                tail->prev=tail->prev->prev;                tail->prev->next=tail;                dMap.erase(temp->key);                _capacity++;            }            dMap[key] = new DoubleListNode(key, value);            dMap[key]->next=head->next;            head->next->prev = dMap[key];            head->next=dMap[key];            dMap[key]->prev=head;            _capacity--;                    }else{            dMap[key]->prev->next = dMap[key]->next;            dMap[key]->next->prev = dMap[key]->prev;            dMap[key]->next=head->next;            head->next->prev = dMap[key];            head->next = dMap[key];            dMap[key]->prev = head;            dMap[key]->val=value;        }    }private:    unordered_map
     
       dMap;    int _capacity;    DoubleListNode* head = new DoubleListNode(-1,-1);    DoubleListNode* tail = new DoubleListNode(-1,-1);};