Problem

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

Solution

Python has OrderedDict which is suitable for this task

from collections import OrderedDict

class LRUCache:

    def __init__(self, capacity: int):
        self._capacity = capacity
        self._cache = OrderedDict()

    def get(self, key: int) -> int:
        # key does not exist in cache
        if key not in self._cache:
            return -1
        
        # move key to the end
        self._cache.move_to_end(key)
        return self._cache[key]

    def put(self, key: int, value: int) -> None:
        # move item to end if it already is in the cache
        if key in self._cache:
            self._cache.move_to_end(key)

        self._cache[key] = value

        # remove first item if the cache exceeds the capacity
        if len(self._cache) > self._capacity:
            self._cache.popitem(last=False)

Result