146. LRU Cache

146. LRU Cache

• Total accepted:109086
• Total submissions:675208
• Difficulty:hard
• Contributors:admin

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 ('ll 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 is reached its capacity, it should invalidate the least recently used item before inserting a new item.

Follow up:
Could 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

We need the data structures:

　　list: List<pair<int, Int>>//record the Order of objects

　　hashmap: Map<int, List<pair<int, int>>::iterator>//mapping the relationship between key and Objects

Must use the iterator, otherwise the splice function won't work

Functions:

Set (key, value)//if the object exists int the map, erase it in the list first, and then push the object from the front. Set M[key] = L.front ();

If the size of map is larger than capacity, erase the object both from list and map;

Get (key)//if the objects do not exist in map, return-1;

The only change was the order in the list.

To_list.splice (position, from_list, iterator)

1 classLRUCache {2  Public:3LRUCache (intcapacity) {4Cap =capacity;5     }6     7     int Get(intkey) {8Auto it =M.find (key);9         if(It = = M.end ())return-1;//Not findTenL.splice (L.begin (), L, it-second);//move it in L to L.begin (); One         returnIt-secondsecond; A     } -      -     voidPutintKeyintvalue) { theAuto it =M.find (key); -         if(It! = M.end ()) L.erase (IT-second); -         //L.push_front (IT-second); - L.push_front (Make_pair (key, value)); +M[key] =L.begin (); -         if(M.size () >cap) { +Auto k = L.rbegin (), first;//to save int type or pointer unsafe because the list to be pop_back later A L.pop_back (); at             //M.erase (k-first); - M.erase (k); -         } -     } - Private: -     intcap; inlist<pair<int,int>> l;//record the order of least recently used -map<int, list<pair<int,int>>::iterator> m;//HashMap must be iterator . to }; +  - /** the * Your LRUCache object would be instantiated and called as such: * * LRUCache obj = new LRUCache (capacity); $ * int param_1 = Obj.get (key);Panax Notoginseng * Obj.put (key,value); -  */

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146. LRU Cache