LRU Cache

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.

这个题用双向linkedlist最好做，同时用hashtable来方便查找到需要的listnode。最左边的代表最近used的，最右边的代表没有used的。

public class LRUCache {
    public int capacity, size;
    public ListNode dummyHead, dummyTail;
    public Dictionary<int,ListNode> dic;
    public LRUCache(int capacity) {
        this.capacity = capacity;
        size = 0;
        dummyHead = new ListNode(0, 0);
        dummyTail = new ListNode(0, 0);
        dummyTail.prev = dummyHead;
        dummyHead.next = dummyTail;
        dic = new Dictionary<int,ListNode>();
    }

    public int Get(int key) {
        if(dic.ContainsKey(key))
        {
             
            var node = dic[key];
            Remove(node);
            AddToHead(node);
            return node.val;
        }
        else return -1;
    }

    public void Set(int key, int value) {
        if(dic.ContainsKey(key))
        {
            var node = dic[key];
            Remove(node);
            node.val = value;
            AddToHead(node);
        }
        else
        {
            if(size >= capacity)
            {
                dic.Remove( dummyTail.prev.key);
                RemoveLast();
            }
            var newNode = new ListNode(key,value);
            AddToHead(newNode);
            dic.Add(key,newNode);
            size++;
        }
    }
    
    
    private void Remove(ListNode node)
    {
        if(node == null) return;
        if(node.prev != null)node.prev.next = node.next;
        if(node.next != null) node.next.prev = node.prev;
    }
    
    private void RemoveLast()
    {
        dummyTail.prev.prev.next = dummyTail;
        dummyTail.prev = dummyTail.prev.prev;
    }
    private void AddToHead(ListNode node)
    {
        if(node == null) return;
        var temp = dummyHead.next;
        dummyHead.next = node;
        node.prev = dummyHead;
        temp.prev = node;
        node.next = temp;
    }
}

public class ListNode
    {
        public int key, val;
        public ListNode prev, next;

        public ListNode(int k, int v)
        {
            key = k;
            val = v;
            prev = null;
            next = null;
        }
    }