[LeetCode 146] LRU Cache

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

// Doubly Linked List node that stores key, value
class Node {
    key: number
    value: number;
    prev: Node = null
    next: Node = null;
    
    constructor(key, value){
        this.key = key;
        this.value = value;
    }
}

class LRUCache {
    private capacity: number
    // stores key to Node
    private cache;
    // left most node, least recently used
    private LRU: Node
    // right most node, most recently used
    private MRU: Node;
    
    constructor(capacity: number) {
        this.capacity = capacity;
        this.cache = {}
        this.LRU = new Node(0, 0)
        this.MRU = new Node(0, 0);
        this.LRU.next = this.MRU;
        this.MRU.prev = this.LRU;
    }

    // remove a node from list
    remove(node: Node){
        const next: Node = node.next, prev: Node = node.prev;
        
        prev.next = next;
        next.prev = prev;
    }

    // add a node before MRU
    add(node: Node){
        const prev: Node = this.MRU.prev, next: Node = this.MRU;
        
        prev.next = node;
        next.prev = node;
        node.prev = prev;
        node.next = next;
    }

    get(key: number): number {
        // if key exists in cache, update node & return the value
        if(key in this.cache){
            this.remove(this.cache[key])
            this.add(this.cache[key])
            return this.cache[key].value;
        }
        
        return -1;
    }

    put(key: number, value: number): void {
        // remove if exists
        if(key in this.cache){
            this.remove(this.cache[key])
        }
        // update cache & list
        this.cache[key] = new Node(key, value);
        this.add(this.cache[key])

        if(Object.keys(this.cache).length > this.capacity){
            const LRUNode: Node = this.LRU.next;
            this.remove(LRUNode)
            delete this.cache[LRUNode.key]
        }
    }
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * var obj = new LRUCache(capacity)
 * var param_1 = obj.get(key)
 * obj.put(key,value)
 */

// Fortunately JS/TS has a very convenient structure called Map, which behaves like an object, with the particularity of respecting insertion order of keys. 
// So we know if we add a key, it will be at the end of the object, and the first inserted key will be at the beginning.

// The "key" (haha) is to be sure to remove keys and re-add it at the end of the map when interacted with.
class LRUCache {
  capacity: number;
  map: Map<number, number>;

  constructor(capacity: number) {
      this.capacity = capacity;
      this.map = new Map();
  }

  get(key: number): number {
    const value = this.map.get(key);

    if (value === undefined) return -1;
          
    this.map.delete(key);
    this.map.set(key, value);
    
    return value;
  }

  put(key: number, value: number): void {
    // remove last element to avoid overflow, only if it does not have 
    // the inserted key is a new key
    if (this.map.size >= this.capacity && !this.map.has(key)) {
        const firstKey = this.map.keys().next().value;
        this.map.delete(firstKey);
    }

    this.map.delete(key);
    this.map.set(key, value);
  }
}