I'm posting my code for a LeetCode problem. If you'd like to review, please do so.

Problem

• 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 theleast recently used item before inserting a new item.

• The cache is initialized with a positive capacity.

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

Accepted Python

class LRUCache:    def __init__(self, capacity: int) -> None:        self.cache = {}        self.capacity = capacity        self.next = {}        self.prev = {}        self.head = 'HEAD'        self.tail = 'TAIL'        self.connect(self.head, self.tail)    def connect(self, node_a: int, node_b: int) -> None:        self.next[node_a], self.prev[node_b] = node_b, node_a    def remove(self, key: int) -> None:        self.connect(self.prev[key], self.next[key])        del(self.prev[key], self.next[key], self.cache[key])    def append(self, key: int, val: int) -> None:        self.cache[key] = val        self.connect(self.prev[self.tail], key)        self.connect(key, self.tail)        if len(self.cache) > self.capacity:            self.remove(self.next[self.head])    def get(self, key: int) -> int:        if key not in self.cache:            return -1        val = self.cache[key]        self.remove(key)        self.append(key, val)        return val    def put(self, key: int, val: int) -> None:        if key in self.cache:            self.remove(key)        self.append(key, val)

LeetCode's Solution (Not for review)

class DLinkedNode():     def __init__(self):        self.key = 0        self.value = 0        self.prev = None        self.next = None            class LRUCache():    def _add_node(self, node):        """        Always add the new node right after head.        """        node.prev = self.head        node.next = self.head.next        self.head.next.prev = node        self.head.next = node    def _remove_node(self, node):        """        Remove an existing node from the linked list.        """        prev = node.prev        new = node.next        prev.next = new        new.prev = prev    def _move_to_head(self, node):        """        Move certain node in between to the head.        """        self._remove_node(node)        self._add_node(node)    def _pop_tail(self):        """        Pop the current tail.        """        res = self.tail.prev        self._remove_node(res)        return res    def __init__(self, capacity):        """        :type capacity: int        """        self.cache = {}        self.size = 0        self.capacity = capacity        self.head, self.tail = DLinkedNode(), DLinkedNode()        self.head.next = self.tail        self.tail.prev = self.head            def get(self, key):        """        :type key: int        :rtype: int        """        node = self.cache.get(key, None)        if not node:            return -1        # move the accessed node to the head;        self._move_to_head(node)        return node.value    def put(self, key, value):        """        :type key: int        :type value: int        :rtype: void        """        node = self.cache.get(key)        if not node:             newNode = DLinkedNode()            newNode.key = key            newNode.value = value            self.cache[key] = newNode            self._add_node(newNode)            self.size += 1            if self.size > self.capacity:                # pop the tail                tail = self._pop_tail()                del self.cache[tail.key]                self.size -= 1        else:            # update the value.            node.value = value            self._move_to_head(node)

Reference

On LeetCode, there is a class usually namedSolution with one or morepublic functions which we are not allowed to rename.

• 146. LRU Cache - Problem

• 146. LRU Cache - Discuss

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