LFU Cache
24. LFU Cache
中文English
LFU (Least Frequently Used) is a famous cache eviction algorithm.
For a cache with capacity k, if the cache is full and need to evict a key in it, the key with the lease frequently used will be kicked out.
Implement set and get method for LFU cache.
Example
Given capacity=3
set(2,2)
set(1,1)
get(2)
>> 2
get(1)
>> 1
get(2)
>> 2
set(3,3)
set(4,4)
get(3)
>> -1
get(2)
>> 2
get(1)
>> 1
get(4)
>> 4
维护一个 map 实现取数据时的O(1)复杂度
维护一个双链表实现更新数据时的O(1)~O(n)复杂度
class LFUCache:
"""
@param: capacity: An integer
"""
class ListNode:
def __init__(self, key, value, next=None, prev=None, visit_count=0):
self.key = key
self.value = value
self.next = next
self.prev = prev
self.visit_count = visit_count
def __init__(self, capacity):
# do intialization if necessary
self.l = 0
self.capacity = capacity
self.maps = {}
self.start = self.ListNode(key=None, value=None)
self.tail = self.ListNode(key=None, value=None)
self.start.next = self.tail
self.tail.prev = self.start
"""
@param: key: An integer
@param: value: An integer
@return: nothing
"""
def set(self, key, value):
# write your code here
node = self.maps.get(key)
if node != None:
# 已存在key 更新visitcount 移动位置
node.value = value
node.visit_count += 1
tmp = node.prev
while node.visit_count >= tmp.visit_count and tmp != self.start:
tmp = tmp.prev
node.next.prev = node.prev
node.prev.next = node.next
node.next = tmp.next
tmp.next.prev = node
tmp.next = node
node.prev = tmp
return
else:
# key不存在 新建一个key
# 进一步分为两种情况 缓存已满和缓存未满
node = self.ListNode(key, value, self.tail, self.tail.prev)
self.tail.prev.next = node
self.tail.prev = node
if self.l < self.capacity:
self.l += 1
else:
invalid_node = node.prev
invalid_key = invalid_node.key
del self.maps[invalid_key]
invalid_node.prev.next = node
node.prev = invalid_node.prev
tmp = node.prev
while node.visit_count >= tmp.visit_count and tmp != self.start:
tmp = tmp.prev
node.next.prev = node.prev
node.prev.next = node.next
node.next = tmp.next
tmp.next.prev = node
tmp.next = node
node.prev = tmp
self.maps[key] = node
"""
@param: key: An integer
@return: An integer
"""
def get(self, key):
# write your code here
node = self.maps.get(key)
if node:
node.visit_count += 1
tmp = node.prev
while node.visit_count >= tmp.visit_count and tmp != self.start:
tmp = tmp.prev
node.next.prev = node.prev
node.prev.next = node.next
node.next = tmp.next
tmp.next.prev = node
tmp.next = node
node.prev = tmp
return node.value
else:
return -1