146. LRU Cache

Problem Link

Description

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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

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Python3

class Node:
 
    def __init__(self, key = None, value = None):
        self.key = key
        self.value = value
        self.prev = self.next = None
 
class LRUCache:
 
    def __init__(self, capacity: int):
        self.size = 0
        self.capacity = capacity
        self.mp = dict()
        self.head = Node()
        self.tail = Node()
        self.head.next = self.tail
        self.tail.prev = self.head
    
    def append(self, node):
        node.prev = self.head
        node.next = self.head.next
        node.next.prev = node
        self.head.next = node
 
    def get(self, key: int) -> int:
        if key not in self.mp: return -1
 
        node = self.mp[key]
        self.pop(node)
        self.append(node)
 
        return node.value
    
    def pop(self, node = None):
        if not node:
            node = self.tail.prev
        
        prev = node.prev
        nxt = node.next
        prev.next = nxt
        nxt.prev = prev
 
        return node
 
    def put(self, key: int, value: int) -> None:
        if key in self.mp:
            node = self.mp[key]
            self.pop(node)
            node.value = value
            self.append(node)
        else:
            if self.size == self.capacity:
                node = self.pop()
                del self.mp[node.key]
                self.size -= 1
            
            node = Node(key, value)
            self.mp[key] = node
            self.append(node)
            self.size += 1
 
# Your LRUCache object will be instantiated and called as such:
# obj = LRUCache(capacity)
# param_1 = obj.get(key)
# obj.put(key,value)