README.md

146. LRU Cache

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.

Solutions (Python)

1. Solution

class ListNode:
    def __init__(self, key=0, val=0, prev=None, next=None):
        self.key = key
        self.val = val
        self.prev = prev
        self.next = next


class LRUCache:

    def __init__(self, capacity: int):
        self.capacity = capacity
        self.keynodes = {}
        self.head = None
        self.tail = None

    def get(self, key: int) -> int:
        if key not in self.keynodes:
            return -1

        self.__move2tail(key)

        return self.keynodes[key].val

    def put(self, key: int, value: int) -> None:
        if len(self.keynodes) == 0:
            self.head = ListNode(key, value)
            self.tail = self.head
            self.keynodes[key] = self.head
        elif key not in self.keynodes:
            self.tail.next = ListNode(key, value, self.tail)
            self.tail = self.tail.next
            self.keynodes[key] = self.tail
            if len(self.keynodes) > self.capacity:
                self.keynodes.pop(self.head.key)
                self.head = self.head.next
                self.head.prev = None
        else:
            self.keynodes[key].val = value
            self.__move2tail(key)

    def __move2tail(self, key: int) -> None:
        if len(self.keynodes) <= 1 or key not in self.keynodes or key == self.tail.key:
            pass
        elif key == self.head.key:
            self.head.next.prev = None
            self.tail.next = self.head
            self.head.prev = self.tail
            self.tail = self.head
            self.head = self.head.next
            self.tail.next = None
        else:
            node = self.keynodes[key]
            node.prev.next = node.next
            node.next.prev = node.prev
            node.prev = self.tail
            self.tail.next = node
            self.tail = node


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