0146-lru-cache¶

Description¶

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

Implement the LRUCache class:

LRUCache(int capacity) Initialize the LRU cache with positive size capacity.
int get(int key) Return the value of the key if the key exists, otherwise return -1.
void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key.

The functions get and put must each run in O(1) average time complexity.

Example 1:

Input
["LRUCache", "put", "put", "get", "put", "get", "put", "get", "get", "get"]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
Output
[null, null, null, 1, null, -1, null, -1, 3, 4]

Explanation
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // cache is {1=1}
lRUCache.put(2, 2); // cache is {1=1, 2=2}
lRUCache.get(1);    // return 1
lRUCache.put(3, 3); // LRU key was 2, evicts key 2, cache is {1=1, 3=3}
lRUCache.get(2);    // returns -1 (not found)
lRUCache.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}
lRUCache.get(1);    // return -1 (not found)
lRUCache.get(3);    // return 3
lRUCache.get(4);    // return 4

Constraints:

1 <= capacity <= 3000
0 <= key <= 10⁴
0 <= value <= 10⁵
At most 2 * 10⁵ calls will be made to get and put.

Solution(Python)¶

class Node:
    def __init__(self,key,value, next_, prev):
        self.key = key
        self.value = value
        self.next = next_
        self.prev = prev 

class DoublyLinkedList:
    def __init__(self):
        self.head = Node(None,None,  None, None)
        self.tail = Node(None,None,  None, None)
        self.head.next = self.tail
        self.tail.prev = self.head
        self.size = 0

    def remove_node(self, node):
        node.prev.next = node.next
        node.next.prev = node.prev
        self.size -= 1

    def insertMRU(self, node):
        tail = self.tail.prev
        tail.next = node
        node.prev = tail
        node.next = self.tail
        self.tail.prev = node
        self.size += 1

    def evictLRU(self):
        lru_node =  self.head.next
        key = lru_node.key
        self.remove_node(lru_node)
        return key
        
    def getSize(self):
        return self.size

class LRUCache:

    def __init__(self, capacity: int):
        self.hashmap = {}
        self.list = DoublyLinkedList()
        self.cap = capacity

    def get(self, key: int) -> int:
        if key not in self.hashmap:
            return -1
        node = self.hashmap[key]
        self.list.remove_node(node)
        self.list.insertMRU(node)
        return node.value
        

    def put(self, key: int, value: int) -> None:
        if key  in self.hashmap:
            node = self.hashmap[key]
            node.value = value
            self.list.remove_node(node)
            self.list.insertMRU(node)
            return 

        if (self.list.getSize() >= self.cap):
            lru_key = self.list.evictLRU()
            del self.hashmap[lru_key]

        node = Node(key, value, None,None)
        self.hashmap[key] = node
        self.list.insertMRU(node)

    

# Your LRUCache object will be instantiated and called as such:
# obj = LRUCache(capacity)
# param_1 = obj.get(key)
# obj.put(key,value)