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
public class LRUCache {
    private int size;
    private int capacity;
    private Dictionary<int, Node> cache = new Dictionary<int, Node>();
    private Node head = new Node();
    private Node tail = new Node();

    public LRUCache(int capacity) {
        this.capacity = capacity;
        head.Next = tail;
        tail.Prev = head;
    }

    public int Get(int key) {
        if (!cache.ContainsKey(key)) {
            return -1;
        }
        Node node = cache[key];
        RemoveNode(node);
        AddToHead(node);
        return node.Val;
    }

    public void Put(int key, int value) {
        if (cache.ContainsKey(key)) {
            Node node = cache[key];
            RemoveNode(node);
            node.Val = value;
            AddToHead(node);
        } else {
            Node node = new Node(key, value);
            cache[key] = node;
            AddToHead(node);
            if (++size > capacity) {
                node = tail.Prev;
                cache.Remove(node.Key);
                RemoveNode(node);
                --size;
            }
        }
    }

    private void RemoveNode(Node node) {
        node.Prev.Next = node.Next;
        node.Next.Prev = node.Prev;
    }

    private void AddToHead(Node node) {
        node.Next = head.Next;
        node.Prev = head;
        head.Next = node;
        node.Next.Prev = node;
    }

    // Node class to represent each entry in the cache.
    private class Node {
        public int Key;
        public int Val;
        public Node Prev;
        public Node Next;

        public Node() {}

        public Node(int key, int val) {
            Key = key;
            Val = val;
        }
    }
}

/**
 * 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);
 */
