Obsidian Notes

9. LRU Cache

3 min read

Description

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 <= 10^4
  • 0 <= value <= 10^5
  • At most 2 * 105 calls will be made to get and put.

Approach

  • LinkedHashMap has a method which returns false by default meaning do not remove any element now we have made it conditional so we will override this method to check if current capacity of Map is greater than allowed capacity or not
  • Capacity argument is initial capacity means this is the initial capacity which is by default 16 meaning we kind of find remainder by dividing by 16 and store the value in this array
  • In case of collision at the same index we store like a linked list
  • load factor means how much capacity to cross here 75% then double the capacity and last argument is to make sure whether you want the order to be based on access or based on insertion (access order true means least accessed towards end I guess)
protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
    return false;  // Default behavior: do not remove any entry
}
class LRUCache {
    private final Map<Integer,Integer> cache;
    private final int capacity;
 
    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.cache = new LinkedHashMap<>(capacity, 0.75f, true) {
            protected boolean removeEldestEntry (Map.Entry<Integer, Integer> oldest) {
                return size() > LRUCache.this.capacity;
            }
        };
    }
    
    public int get(int key) {
        return cache.getOrDefault(key,-1);
    }
    
    public void put(int key, int value) {
        cache.put(key, value);
    }
}
 
/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache obj = new LRUCache(capacity);
 * int param_1 = obj.get(key);
 * obj.put(key,value);
 */

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