LeetCode-in-All

146. LRU Cache

Medium

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

Implement the LRUCache class:

• LRUCache(int capacity) Initialize the LRU cache with positive size capacity.
• int get(int key) Return the value of the key if the key exists, otherwise return -1.
• void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key.

The functions get and put must each run in O(1) average time complexity.

Example 1:

Input [“LRUCache”, “put”, “put”, “get”, “put”, “get”, “put”, “get”, “get”, “get”] [[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]

Output: [null, null, null, 1, null, -1, null, -1, 3, 4]

Explanation:

LRUCache lRUCache = new LRUCache(2);

lRUCache.put(1, 1); // cache is {1=1}

lRUCache.put(2, 2); // cache is {1=1, 2=2}

lRUCache.get(1); // return 1

lRUCache.put(3, 3); // LRU key was 2, evicts key 2, cache is {1=1, 3=3}

lRUCache.get(2); // returns -1 (not found)

lRUCache.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}

lRUCache.get(1); // return -1 (not found)

lRUCache.get(3); // return 3

lRUCache.get(4); // return 4

Constraints:

• 1 <= capacity <= 3000
• 0 <= key <= 104
• 0 <= value <= 105
• At most 2 * 105 calls will be made to get and put.

Solution

(define lru-cache%
  (class object%
    (super-new)
    
    (init-field capacity)
    
    (define head #f)
    (define tail #f)
    (define cache (make-hash))
    
    (define/private (move-to-head node)
      (when (not (eq? node head))
        (when (eq? node tail)
          (set! tail (hash-ref node 'prev))
          (when tail (hash-set! tail 'next #f)))
        (let ((prev (hash-ref node 'prev #f))
              (next (hash-ref node 'next #f)))
          (when prev (hash-set! prev 'next next))
          (when next (hash-set! next 'prev prev))
          (hash-set! node 'prev #f)
          (hash-set! node 'next head)
          (when head (hash-set! head 'prev node))
          (set! head node))))
    
    (define/public (get key)
      (let ((node (hash-ref cache key #f)))
        (if node
            (begin (move-to-head node)
                   (hash-ref node 'value))
            -1)))
    
    (define/public (put key value)
      (let ((node (hash-ref cache key #f)))
        (if node
            (begin
              (hash-set! node 'value value)
              (move-to-head node))
            (begin
              (when (>= (hash-count cache) capacity)
                (when tail
                  (hash-remove! cache (hash-ref tail 'key))
                  (set! tail (hash-ref tail 'prev #f))
                  (when tail (hash-set! tail 'next #f))))
              (let ((new-node (make-hash)))
                (hash-set! new-node 'key key)
                (hash-set! new-node 'value value)
                (hash-set! new-node 'prev #f)
                (hash-set! new-node 'next head)
                (when head (hash-set! head 'prev new-node))
                (set! head new-node)
                (unless tail (set! tail new-node))
                (hash-set! cache key new-node))))))))

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