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
分析: 一个比较好的设计数据结构的例子， 使用unordered_map 和list 容器， list 存放最近查询的节点 很巧妙的是map 中存放<int, list<pair<int, int>>::iterator> list中存放 list<pair<int, int>>， 这样两者互相访问都是O(1)
class LRUCache { public: LRUCache(int capacity):_capacity(capacity) { } int get(int key) { auto iter = Cache_Map.find(key); if(iter == Cache_Map.end()) return -1; int value = iter->second->second; Cache_List.splice(Cache_List.begin(),Cache_List, iter->second); return value; } void put(int key, int value) { auto iter = Cache_Map.find(key); if(iter != Cache_Map.end()){ iter->second->second = value; Cache_List.splice(Cache_List.begin(),Cache_List, iter->second); return; } if(Cache_Map.size() == _capacity){ int del = Cache_List.back().first; cout << "delete "<< del << endl; Cache_List.pop_back(); Cache_Map.erase(del); } Cache_List.emplace_front(key,value); // cout << "print the map"<<endl; // for(auto it = Cache_Map.begin(); it!=Cache_Map.end(); it++) // cout << it->first << " -> "<< it->second->second<<endl; // cout << "print the list"<<endl; // for(auto it = Cache_List.begin(); it!=Cache_List.end(); it++) // cout << it->first << " -> "<< it->second<<endl; Cache_Map[key] = Cache_List.begin(); } private: int _capacity; unordered_map<int, list<pair<int, int>>::iterator> Cache_Map; list<pair<int, int>> Cache_List; }; /** * 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); */
原文：http://www.cnblogs.com/willwu/p/6403779.html