type LRUCache struct {
    size int
    capacity int
    cache map[int]*DLinkedNode
    head, tail *DLinkedNode
}

type DLinkedNode struct {
    key, val int
    prev, next *DLinkedNode
}

func initDLinkedNode(key, val int) *DLinkedNode {
    return &DLinkedNode{
        key: key,
        val: val,
    }
}

func Constructor(capacity int) LRUCache {
    l := LRUCache{
        cache: map[int]*DLinkedNode{},
        head: initDLinkedNode(0, 0),
        tail: initDLinkedNode(0, 0),
        capacity: capacity,
    }
    l.head.next = l.tail
    l.tail.prev = l.head
    return l
}

func (this *LRUCache) addToHead(node *DLinkedNode) {
    node.prev = this.head
    node.next = this.head.next
    this.head.next.prev = node
    this.head.next = node
}

func (this *LRUCache) removeNode(node *DLinkedNode) {
	// 将节点从链表中单独抽出来
    node.prev.next = node.next
    node.next.prev = node.prev
}

func (this *LRUCache) moveToHead(node *DLinkedNode) {
    this.removeNode(node)
    this.addToHead(node)
}

func (this *LRUCache) removeTail() *DLinkedNode {
    node := this.tail.prev
    this.removeNode(node)
    delete(this.cache, node.key)
    return node
}

// 通过cache的map关系，找到对应的值，该值存储在node的val属性中。
func (this *LRUCache) Get(key int) int {
	// 如果key是不存在的，那就返回-1
    if _, ok := this.cache[key]; !ok {
        return -1
    }
    node := this.cache[key]
    this.moveToHead(node)
    return node.val
}


func (this *LRUCache) Put(key int, val int)  {
    if _, ok := this.cache[key]; !ok {
        node := initDLinkedNode(key, val)
        this.cache[key] = node
        this.addToHead(node)
        this.size++
        if this.size > this.capacity {
            removed := this.removeTail()
            delete(this.cache, removed.key)
            this.size--
        }
    } else {
        node := this.cache[key]
        node.val = val
        this.moveToHead(node)
    }
}