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146. LRU Cache
题目
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
解题思路
LRU缓存利用了这样的一种思想。LRU是Least Recently Used 的缩写，翻译过来就是“最近最少使用”，也就是说，LRU缓存把最近最少使用的数据移除，让给最新读取的数据。而往往最常读取的，也是读取次数最多的，所以，利用LRU缓存，我们能够提高系统的performance.
见程序注释
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Types ¶
type LRUCache ¶
type LRUCache struct {
// contains filtered or unexported fields
}
LRUCache contains a hash map and a doubly linked list
func (*LRUCache) Get ¶
Get a list node from the hash map.
func (c *LRUCache) Get(key int) int {
// check if list node exists
if node, ok := c.m[key]; ok {
val := node.Value.(*list.Element).Value.(Pair).value
// move node to front
c.l.MoveToFront(node)
return val
}
return -1
}
// Put key and value in the LRUCache
func (c *LRUCache) Put(key int, value int) {
// check if list node exists
if node, ok := c.m[key]; ok {
// move the node to front
c.l.MoveToFront(node)
// update the value of a list node
node.Value.(*list.Element).Value = Pair{key: key, value: value}
} else {
// delete the last list node if the list is full
if c.l.Len() == c.cap {
// get the key that we want to delete
idx := c.l.Back().Value.(*list.Element).Value.(Pair).key
// delete the node pointer in the hash map by key
delete(c.m, idx)
// remove the last list node
c.l.Remove(c.l.Back())
}
// initialize a list node
node := &list.Element{
Value: Pair{
key: key,
value: value,
},
}
// push the new list node into the list
ptr := c.l.PushFront(node)
// save the node pointer in the hash map
c.m[key] = ptr
}
}
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