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 the least 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 named Solution with one or more public functions which we are not allowed to rename.
