其实也米有很难……只是c++11的api这么好用的吗

Design and implement a data structure for Least Recently Used (LRU) cache. 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 1
cache.put(3, 3);    // evicts key 2
cache.get(2);       // returns -1 (not found)
cache.put(4, 4);    // evicts key 1
cache.get(1);       // returns -1 (not found)
cache.get(3);       // returns 3
cache.get(4);       // returns 4

class LRUCache {
public:LRUCache(int capacity) {size = capacity;}int get(int key) {auto it = hash.find(key);if (it != hash.end()){ // find// it->second移到cache.begin()前面的位置cache.splice(cache.begin(), cache, it->second);  return it->second->second;  // return value
        }elsereturn -1;}void put(int key, int value) {auto it = hash.find(key);if (it != hash.end()){   // findit->second->second = value;return cache.splice(cache.begin(), cache, it->second);}cache.insert(cache.begin(), make_pair(key, value));hash[key] = cache.begin();if (cache.size() > size){hash.erase(cache.back().first);cache.pop_back();}}private:// C++ STL中，哈希表对应的容器是 unordered_map（since C++ 11）。根据 C++ 11 标准的推荐，用 unordered_map 代替 hash_map.// STL中，map 对应的数据结构是 红黑树。红黑树是一种近似于平衡的二叉查找树，里面的数据是有序的。在红黑树上做查找操作的时间复杂度为 O(logN)。而 unordered_map 对应 哈希表，哈希表的特点就是查找效率高，时间复杂度为常数级别 O(1)， 而额外空间复杂度则要高出许多。所以对于需要高效率查询的情况，使用 unordered_map 容器。而如果对内存大小比较敏感或者数据存储要求有序的话，则可以用 map 容器。// Lists将元素按顺序储存在链表中. 与 向量(vectors)相比, 它允许快速的插入和删除，但是随机访问却比较慢.// hash表：存储key//unordered_map<int, list<pair(int, int)>::iterator> hash;unordered_map<int, list<pair<int, int>>::iterator> hash;// 链表：缓存区list<pair<int, int>> cache;// 缓存区大小int size;};/*** Your LRUCache object will be instantiated and called as such:* LRUCache* obj = new LRUCache(capacity);* int param_1 = obj->get(key);* obj->put(key,value);*/