思路：
双向链表+哈希表。
实现：
1 class node 2 { 3 public: 4 int key, val; 5 node* prev, *next; 6 node(int k, int v): key(k), val(v) { prev = NULL; next = NULL; } 7 }; 8 class LRUCache 9 { 10 int _capacity = 0; 11 unordered_map<int, node*> mp; 12 node* dummy_head = NULL, *dummy_tail = NULL; 13 int cnt = 0; 14 public: 15 LRUCache(int capacity): _capacity(capacity) 16 { 17 dummy_head = new node(-1, -1); 18 dummy_head->next = new node(-1, -1); 19 dummy_tail = dummy_head->next; 20 dummy_tail->prev = dummy_head; 21 } 22 23 void move_back(node* tmp) 24 { 25 node* p = tmp->prev, *n = tmp->next; 26 p->next = tmp->next; n->prev = tmp->prev; 27 node* fuck = dummy_tail->prev; 28 fuck->next = tmp; 29 tmp->prev = fuck; 30 dummy_tail->prev = tmp; 31 tmp->next = dummy_tail; 32 } 33 34 void erase() 35 { 36 node* tmp = dummy_head->next, *n = tmp->next; 37 dummy_head->next = n; 38 n->prev = dummy_head; 39 tmp->next = tmp->prev = NULL; 40 mp.erase(tmp->key); 41 delete(tmp); tmp = NULL; 42 } 43 44 int get(int key) 45 { 46 if (mp.count(key)) 47 { 48 node* tmp = mp[key]; 49 move_back(tmp); 50 return mp[key]->val; 51 } 52 return -1; 53 } 54 55 void put(int key, int value) 56 { 57 if (mp.count(key)) 58 { 59 node* tmp = mp[key]; 60 tmp->val = value; 61 move_back(tmp); 62 } 63 else 64 { 65 if (cnt == _capacity) { erase(); cnt--; } 66 node* p = dummy_tail->prev; 67 node* tmp = new node(key, value); 68 p->next = tmp; tmp->next = dummy_tail; 69 dummy_tail->prev = tmp; tmp->prev = p; 70 mp[key] = tmp; 71 cnt++; 72 } 73 } 74 }; 75 76 /** 77 * Your LRUCache object will be instantiated and called as such: 78 * LRUCache* obj = new LRUCache(capacity); 79 * int param_1 = obj->get(key); 80 * obj->put(key,value); 81 */