Question

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

Solution

Use doubly Linked List to track access history, and use dict to track key-value mapping. The value in dict should be node in the doubly linked list.

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

class LRUCache(object):

    def __init__(self, capacity):
        """
        :type capacity: int
        """
        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):
        """
        :type key: int
        :rtype: int
        """
        if key in self.dic:
            n = self.dic[key]
            self._remove(n)
            self._add(n)
            return n.val
        return -1



    def put(self, key, value):
        """
        :type key: int
        :type value: int
        :rtype: void
        """
        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):
        """
        Remove the node from the doubly-linked-list
        """
        p = node.prev
        n = node.next
        p.next = n
        n.prev = p

    def _add(self, node):
        """
        Insert the node to tail
        """
        p = self.tail.prev
        p.next = node
        self.tail.prev = node
        node.prev = p
        node.next = self.tail

# Your LRUCache object will be instantiated and called as such:
# obj = LRUCache(capacity)
# param_1 = obj.get(key)
# obj.put(key,value)