LRU Cache

Driver Code

obj = LRUCache(capacity)
param_1 = obj.get(key)
obj.put(key,value)

Solution

Both solutions below take constant time.

Complexity

time: $O(1)$ (all operations)
space: $O(n)$

Cheating w/ Built-in OrderDict

from collections import OrderedDict

class LRUCache:
    def __init__(self, capacity: int):
        self.cap = capacity
        self.dct = OrderedDict()

    def get(self, key: int) -> int:
        if key not in self.dct:
            return -1
        # update order
        self.dct.move_to_end(key, last=True)
        # get from od
        return self.dct[key]

    def put(self, key: int, value: int) -> None:
        # remove old entry if present
        if key in self.dct:
            self.dct.move_to_end(key, last=True)
        # put in od
        self.dct[key] = value
        # remove lru if exceed capacity
        if len(self.dct) > self.cap:
            self.dct.popitem(last=False)

HashMap & Doubly Linked List

dct is a hashmap that maps from key to ListNode, while head and tail define a doubly linked list that makes adding, moving, and removing nodes $O(1)$.

# Doubly Linked List node
class ListNode:
    def __init__(self, key, val):
        self.key = key
        self.val = val
        self.prev = None
        self.next = None

class LRUCache:
    def __init__(self, capacity: int):
        self.cap = capacity
        self.dct = {} # key -> ListNode
        # head and tail are sentinel nodes
        self.head = ListNode(-1, -1)
        self.tail = ListNode(-1, -1)
        self.head.next = self.tail
        self.tail.prev = self.head

    def add_node(self, node): # add node to end of ll
        # update tail & node
        self.tail.prev.next = node
        node.prev = self.tail.prev
        node.next = self.tail
        self.tail.prev = node
        
    def del_node(self, node): # remove node
        # if (not node.prev):
        #     print('oh no')
        node.prev.next = node.next
        node.next.prev = node.prev
        node.next = None
        node.prev = None

    def get(self, key: int) -> int:
        # if key not in hashmap, return -1
        if key not in self.dct:
            return -1
        # find node, remove in ll, add to end
        node = self.dct[key]
        self.del_node(node)
        self.add_node(node)
        return node.val

    def put(self, key: int, value: int) -> None:
        # if key exists, remove from ll
        if key in self.dct:
            node = self.dct[key]
            self.del_node(node)
        
        # update value, add to ll
        node = ListNode(key, value)
        self.dct[key] = node
        self.add_node(node)

        # remove lru if exceeds capacity
        if len(self.dct) > self.cap:
            lru = self.head.next
            self.del_node(lru)
            del self.dct[lru.key]