LRUcache.md

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

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1. put(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.

Follow up: Could you do both operations in O(1) time complexity?

Example:

LRUCache cache = new LRUCache( 2 /* capacity */ );

cache.put(1, 1);
cache.put(2, 2);
cache.get(1);       // returns 1
cache.put(3, 3);    // evicts key 2
cache.get(2);       // returns -1 (not found)
cache.put(4, 4);    // evicts key 1
cache.get(1);       // returns -1 (not found)
cache.get(3);       // returns 3
cache.get(4);       // returns 4

class Node:
  def __init__(self, k, v):
      self.key = k
      self.val = v
      self.prev = None
      self.next = None

class LRUCache:
  def __init__(self, capacity):
      self.capacity = capacity
      self.dic = dict()
      self.head = Node(0, 0)
      self.tail = Node(0, 0)
      self.head.next = self.tail
      self.tail.prev = self.head

  def get(self, key):
      if key in self.dic:
          n = self.dic[key]
          self._remove(n)
          self._add(n)
          return n.val
      return -1

  def set(self, key, value):
      if key in self.dic:
          self._remove(self.dic[key])
      n = Node(key, value)
      self._add(n)
      self.dic[key] = n
      if len(self.dic) > self.capacity:
          n = self.head.next
          self._remove(n)
          del self.dic[n.key]

  def _remove(self, node):
      p = node.prev
      n = node.next
      p.next = n
      n.prev = p

  def _add(self, node):
      p = self.tail.prev
      p.next = node
      self.tail.prev = node
      node.prev = p
      node.next = self.tail

Little Easier to read with comments

class LRUCache(object):

    def __init__(self, capacity):
        self.capacity = capacity
        self.head = LinkedNode(None,'head')
        self.tail = LinkedNode(None,'tail')
        self.head.next = self.tail # tail being most recent
        self.tail.prev = self.head # head being oldest
        self.data = {}

    def deleteNode(self,node):
        assert(node is not self.head and node is not self.tail)
        del self.data[node.key]
        node.prev.next = node.next
        node.next.prev = node.prev
        del node

    def get(self,key):
        if key not in self.data:
            return -1
        node = self.data[key]
        # take the node out
        node.prev.next = node.next
        node.next.prev = node.prev
        # insert into most recent position
        self.insertNew(node)
        return node.value

    def put(self, key, value):
        # remove old value if present
        if key in self.data:
            self.deleteNode(self.data[key])

        # create new node
        newNode = LinkedNode(key,value)
        self.data[key] = newNode

        # if over limit, delete oldest node
        if len(self.data)>self.capacity:
            self.deleteNode(self.head.next)

        self.insertNew(newNode)

    def insertNew(self,newNode):
        # insert new node into last position    
        last = self.tail.prev
        last.next = newNode
        self.tail.prev = newNode
        newNode.next = self.tail
        newNode.prev = last