class Node(object):
    def __init__(self, key=None, value=None, next=None, prev=None):
        self.key = key
        self.value = value
        self.next = next
        self.prev = prev
class LRUCache(object):
    def __init__(self, capacity):
        """
        :type capacity: int
        """
        self.capacity = capacity
        # single linked list with a head node
        # always put new node to the tail
        # also move the revisted node to the tail
        self.head = Node()
        self.tail = self.head
        self.head.next = self.tail
        # <key, node.prev>
        self.hash_table = {}
    def pop_front(self):
        del self.hash_table[self.head.next.key]
        p_next = self.head.next.next
        self.head.next = p_next
        # update the reference for new front node
        self.hash_table[self.head.next.key] = self.head
    def append(self, node):
        self.hash_table[node.key] = self.tail
        self.tail.next = node
        self.tail = node
    def move_to_end(self, prev):
        node = prev.next
        if node == self.tail:
            return
        # disconnect node
        prev.next = node.next
        node.next = None
        self.hash_table[prev.next.key] = prev
        # append node
        self.append(node)
    def get(self, key):
        """
        :type key: int
        :rtype: int
        """
        if key not in self.hash_table:
            return -1
        prev = self.hash_table[key]
        val = prev.next.value
        self.move_to_end(prev)
        return val
    def put(self, key, value):
        """
        :type key: int
        :type value: int
        :rtype: void
        """
        if key in self.hash_table:
            prev = self.hash_table[key]
            prev.next.value = value
            self.move_to_end(prev)
        else:
            self.append(Node(key, value))
            if len(self.hash_table) > self.capacity:
                self.pop_front()