struct CacheNode {
  int key;      // 键
  int value;    // 值
  CacheNode *pre, *next;  // 节点的前驱、后继指针
  CacheNode(int k, int v) : key(k), value(v), pre(NULL), next(NULL) {}
};

LRUCache(int capacity)
{
  size = capacity;      // 容量
  head = NULL;          // 链表头指针
  tail = NULL;          // 链表尾指针
}

void remove(CacheNode *node)
{
  if (node -> pre != NULL)
  {
    node -> pre -> next = node -> next;
  }
  else
  {
    head = node -> next;
  }
  if (node -> next != NULL)
  {
    node -> next -> pre = node -> pre;
  }
  else
  {
    tail = node -> pre;
  }
}

void setHead(CacheNode *node)
{
  node -> next = head;
  node -> pre = NULL;

  if (head != NULL)
  {
    head -> pre = node;
  }
  head = node;
  if (tail == NULL)
  {
    tail = head;
  }
}

int get(int key)
{
  map<int, CacheNode *>::iterator it = mp.find(key);
  if (it != mp.end())
  {
    CacheNode *node = it -> second;
    remove(node);
    setHead(node);
    return node -> value;
  }
  else
  {
    return -1;
  }
}

void set(int key, int value)
{
  map<int, CacheNode *>::iterator it = mp.find(key);
  if (it != mp.end())
  {
    CacheNode *node = it -> second;
    node -> value = value;
    remove(node);
    setHead(node);
  }
  else
  {
    CacheNode *newNode = new CacheNode(key, value);
    if (mp.size() >= size)
    {
      map<int, CacheNode *>::iterator iter = mp.find(tail -> key);
      remove(tail);
      mp.erase(iter);
    }
    setHead(newNode);
    mp[key] = newNode;
  }
}