Oleksii Trekhleb | Javascript algorithms (Least recently used (LRU) cache)
This is a series of books diving deep into the core mechanisms of the JavaScript language.
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A Least Recently Used (LRU) Cache organizes items in order of use, allowing you to quickly identify which item hasn’t been used for the longest amount of time.
Picture a clothes rack, where clothes are always hung up on one side. To find the least-recently used item, look at the item on the other end of the rack.
The problem statement
Implement the LRUCache class:
LRUCache(int capacity)
Initialize the LRU cache with positive sizecapacity
.int get(int key)
Return the value of thekey
if thekey
exists, otherwise returnundefined
.void set(int key, int value)
Update the value of thekey
if thekey
exists. Otherwise, add thekey-value
pair to the cache. If the number of keys exceeds thecapacity
from this operation, evict the least recently used key.
The functions get()
and set()
must each run in O(1)
average time complexity.
Implementation
Version 1: Doubly Linked List + Hash Map
See the LRUCache
implementation example in LRUCache.js. The solution uses a HashMap
for fast O(1)
(in average) cache items access, and a DoublyLinkedList
for fast O(1)
(in average) cache items promotions and eviction (to keep the maximum allowed cache capacity).
Made with okso.app
You may also find more test-case examples of how the LRU Cache works in LRUCache.test.js file.
Version 2: Ordered Map
The first implementation that uses doubly linked list is good for learning purposes and for better understanding of how the average O(1)
time complexity is achievable while doing set()
and get()
.
However, the simpler approach might be to use a JavaScript Map object. The Map
object holds key-value pairs and remembers the original insertion order of the keys. We can use this fact in order to keep the recently-used items in the end of the map by removing and re-adding items. The item at the beginning of the Map
is the first one to be evicted if cache capacity overflows. The order of the items may checked by using the IterableIterator
like map.keys()
.
See the LRUCacheOnMap
implementation example in LRUCacheOnMap.js.
You may also find more test-case examples of how the LRU Cache works in LRUCacheOnMap.test.js file.
Complexities
Average | |
---|---|
Space | O(n) |
Get item | O(1) |
Set item | O(1) |
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