LRU Cache
<p>Design a data structure that follows the constraints of a <strong><a href="https://en.wikipedia.org/wiki/Cache_replacement_policies#LRU" target="_blank">Least Recently Used (LRU) cache</a></strong>.</p>

<p>Implement the <code>LRUCache</code> class:</p>

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

<p>The functions <code>get</code> and <code>put</code> must each run in <code>O(1)</code> average time complexity.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input</strong>
[&quot;LRUCache&quot;, &quot;put&quot;, &quot;put&quot;, &quot;get&quot;, &quot;put&quot;, &quot;get&quot;, &quot;put&quot;, &quot;get&quot;, &quot;get&quot;, &quot;get&quot;]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
<strong>Output</strong>
[null, null, null, 1, null, -1, null, -1, 3, 4]

<strong>Explanation</strong>
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
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= capacity &lt;= 3000</code></li>
	<li><code>0 &lt;= key &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= value &lt;= 10<sup>5</sup></code></li>
	<li>At most <code>2 * 10<sup>5</sup></code> calls will be made to <code>get</code> and <code>put</code>.</li>
</ul>
Design; Hash Table; Linked List; Doubly-Linked List
/**
 * @param {number} capacity
 */
var LRUCache = function (capacity) {
    this.size = 0;
    this.capacity = capacity;
    this.cache = new Map();
    this.head = new Node(0, 0);
    this.tail = new Node(0, 0);
    this.head.next = this.tail;
    this.tail.prev = this.head;
};

/**
 * @param {number} key
 * @return {number}
 */
LRUCache.prototype.get = function (key) {
    if (!this.cache.has(key)) {
        return -1;
    }
    const node = this.cache.get(key);
    this.removeNode(node);
    this.addToHead(node);
    return node.val;
};

/**
 * @param {number} key
 * @param {number} value
 * @return {void}
 */
LRUCache.prototype.put = function (key, value) {
    if (this.cache.has(key)) {
        const node = this.cache.get(key);
        this.removeNode(node);
        node.val = value;
        this.addToHead(node);
    } else {
        const node = new Node(key, value);
        this.cache.set(key, node);
        this.addToHead(node);
        if (++this.size > this.capacity) {
            const nodeToRemove = this.tail.prev;
            this.cache.delete(nodeToRemove.key);
            this.removeNode(nodeToRemove);
            --this.size;
        }
    }
};

LRUCache.prototype.removeNode = function (node) {
    if (!node) return;
    node.prev.next = node.next;
    node.next.prev = node.prev;
};

LRUCache.prototype.addToHead = function (node) {
    node.next = this.head.next;
    node.prev = this.head;
    this.head.next.prev = node;
    this.head.next = node;
};

/**
 * @constructor
 * @param {number} key
 * @param {number} val
 */
function Node(key, val) {
    this.key = key;
    this.val = val;
    this.prev = null;
    this.next = null;
}

/**
 * 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)
 */
