146. LRU Cache

Medium

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

Implement the LRUCache class:

• LRUCache(int capacity) Initialize the LRU cache with positive size capacity.
• int get(int key) Return the value of the key if the key exists, otherwise return -1.
• void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key.

The functionsget and put must each run in O(1) average time complexity.

NOT_IN_CACHE = -1


class LRUCache:
    def __init__(self, capacity: int):
        self.capacity = capacity
        self.key_to_node = {}
        self.list = LinkedList()

    def get(self, key: int) -> int:
        if key not in self.key_to_node:
            return NOT_IN_CACHE

        node = self.key_to_node[key]
        self.list.delete(node)
        self.list.insert_at_head(node)
        return node.val

    def put(self, key: int, value: int) -> None:
        if key in self.key_to_node:
            node = self.key_to_node[key]
            node.val = value
            self.list.delete(node)
            self.list.insert_at_head(node)
            return

        if len(self.key_to_node) >= self.capacity:
            node = self.list.tail.prev
            del self.key_to_node[node.key]
            self.list.delete(node)

        node = Node(key, value)
        self.list.insert_at_head(node)
        self.key_to_node[key] = node


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


class LinkedList:
    def __init__(self):
        self.head = Node("head", "head")
        self.tail = Node("tail", "tail")
        self.head.next = self.tail
        self.tail.prev = self.head

    def insert_at_head(self, node):
        node.next = self.head.next
        self.head.next.prev = node
        self.head.next = node
        node.prev = self.head

    def delete(self, node):
        node.prev.next = node.next
        node.next.prev = node.prev