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.
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
Python Solution:
class DoublyLinkedList(object):
def __init__(self, key, val):
self.key = key
self.val = val
self.pre = None
self.next = None
class LRUCache(object):
def __init__(self, capacity):
"""
:type capacity: int
"""
self.capacity = capacity
self.size = 0
self.head = DoublyLinkedList(0, 0)
self.tail = DoublyLinkedList(0, 0)
self.head.next, self.tail.pre = self.tail, self.head
self.d = {}
def get(self, key):
"""
:type key: int
:rtype: int
"""
if key not in self.d: return -1
node = self.d[key]
node.pre.next, node.next.pre = node.next, node.pre
self.head.next.pre, self.head.next, node.next, node.pre = node, node, self.head.next, self.head
return node.val
def put(self, key, value):
"""
:type key: int
:type value: int
:rtype: void
"""
if key in self.d:
node = self.d[key]
node.val = value
node.pre.next, node.next.pre = node.next, node.pre
elif self.size == self.capacity:
node = self.tail.pre
del self.d[node.key]
node.key = key
node.val = value
self.d[key] = node
node.pre.next, node.next.pre = node.next, node.pre
else:
node = DoublyLinkedList(key, value)
self.d[key] = node
self.size += 1
self.head.next.pre, self.head.next, node.next, node.pre = node, node, self.head.next, self.head
# Your LRUCache object will be instantiated and called as such:
# obj = LRUCache(capacity)
# param_1 = obj.get(key)
# obj.put(key,value)
Summary:
- doubly linked list + hash table
LeetCode: 146. LRU Cache
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