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146-lru-cache.py
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146-lru-cache.py
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# use nodes to store as references for keys, doubly linked list structure
class Node:
def __init__(self, key, value):
self.value = value
self.key = key
self.prev = None
self.next = None
class LRUCache:
def __init__(self, capacity: int):
self.capacity = capacity
self.count = 0
# create head and tail markers
self.head = Node(0,0)
self.tail = Node(0,0)
self.head.next, self.tail.prev = self.tail, self.head
# map for getting Node from key
self.map = {}
def get(self, key: int) -> int:
# return -1 if key does not exist
if key not in self.map:
return -1
node = self.map[key]
# if node not already at end of DLL, move to end
if node.next != self.tail:
# take node out of DLL
node.prev.next, node.next.prev = node.next, node.prev
# append to end of DLL
prev = self.tail.prev
prev.next, node.prev = node, prev
node.next, self.tail.prev = self.tail, node
# return value
return node.value
def put(self, key: int, value: int) -> None:
# if key already exists, remove node from DLL and update value
if key in self.map:
node = self.map[key]
node.prev.next, node.next.prev = node.next, node.prev
node.value = value
self.map[key] = node
# or else instantiate new Node, increase count
else:
node = Node(key, value)
self.map[key] = node
self.count += 1
# append to end of DLL
prev = self.tail.prev
prev.next, node.prev = node, prev
node.next, self.tail.prev = self.tail, node
# if count is greater than capacity, discard LRU
if self.count > self.capacity:
lru = self.head.next
self.head.next, lru.next.prev = lru.next, self.head
del self.map[lru.key]
self.count -= 1
# Your LRUCache object will be instantiated and called as such:
# obj = LRUCache(capacity)
# param_1 = obj.get(key)
# obj.put(key,value)