[LeetCode] LRU Cache, Solution

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

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

[Thoughts]

首先，对于cache，如果希望有O(1)的查找复杂度，肯定要用hashmap来保存key和对象的映射。对于LRU而言，问题在于如何用O(1)解决cache entry的替换问题。

简单的说，cache的存储是一个链表的话，那么只要保证从头到尾的顺序就是cache从新到旧的顺序就好了，对于任何一个节点，如果被访问了，那么就将该节点移至头部。如果cache已满，那么就把尾部的删掉，从头部插入新节点。

所以，需要用到两个数据结构

1. hashmap， 保存key和对象位置的映射

2. list，保存对象新旧程度的序列。不一定是list，也可以用vector，不过list的好处是已经实现了头尾操作的api，vector的话，还要自己写，麻烦。

 

[Code]

 1 class LRUCache{
 2 public:
 3     struct CacheEntry
 4     {
 5     public:
 6         int key;
 7         int value;
 8         CacheEntry(int k, int v) :key(k), value(v) {}
 9     };
10 
11     LRUCache(int capacity) {
12         m_capacity = capacity;
13     }
14 
15     int get(int key) {
16         if (m_map.find(key) == m_map.end())
17             return -1;
18 
19         MoveToHead(key);
20         return m_map[key]->value;
21     }
22 
23     void set(int key, int value) {
24         if (m_map.find(key) == m_map.end())
25         {
26             CacheEntry newItem(key, value);
27             if (m_LRU_cache.size() >= m_capacity)
28             {
29                 //remove from tail
30                 m_map.erase(m_LRU_cache.back().key);
31                 m_LRU_cache.pop_back();                
32             }
33 
34             // insert in head.
35             m_LRU_cache.push_front(newItem);
36             m_map[key] = m_LRU_cache.begin();
37             return;
38         }
39 
40         m_map[key]->value = value;
41         MoveToHead(key);
42     }
43 
44 private:
45     unordered_map<int, list<CacheEntry>::iterator> m_map;
46     list<CacheEntry> m_LRU_cache;
47     int m_capacity;
48 
49     void MoveToHead(int key) 
50     {
51         //Move key from current location to head
52         auto updateEntry = *m_map[key];
53         m_LRU_cache.erase(m_map[key]);
54         m_LRU_cache.push_front(updateEntry);
55         m_map[key] = m_LRU_cache.begin();
56     }
57 
58 };