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146. LRU Cache

Medium

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

public class LRUCache {
    private readonly int capacity;
    private readonly Dictionary<int, LinkedListNode<CacheItem>> cacheMap;
    private readonly LinkedList<CacheItem> cacheList;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        cacheMap = new Dictionary<int, LinkedListNode<CacheItem>>(capacity);
        cacheList = new LinkedList<CacheItem>();
    }
    
    public int Get(int key) {
        if (cacheMap.TryGetValue(key, out var node)) {
            cacheList.Remove(node);
            cacheList.AddFirst(node);
            return node.Value.Value;
        }
        return -1;
    }
    
    public void Put(int key, int value) {
        if (cacheMap.TryGetValue(key, out var node)) {
            node.Value.Value = value;
            cacheList.Remove(node);
            cacheList.AddFirst(node);
        } else {
            if (cacheMap.Count >= capacity) {
                var lastNode = cacheList.Last;
                cacheMap.Remove(lastNode.Value.Key);
                cacheList.RemoveLast();
            }
            var newNode = new LinkedListNode<CacheItem>(new CacheItem(key, value));
            cacheMap.Add(key, newNode);
            cacheList.AddFirst(newNode);
        }
    }

    private class CacheItem {
        public int Key { get; }
        public int Value { get; set; }

        public CacheItem(int key, int value) {
            Key = key;
            Value = value;
        }
    }
}

public class KeyNode {
    private int _key;
    private int _val;
    private KeyNode _next;

    public int key { 
        get { return _key; }
        set { _key = value; } 
    }

    public int val {
        get { return _val; }
        set { _val = value; }
    }

    public KeyNode next {
        get { return _next; }
        set { _next = value; }
    }

    public KeyNode() {
        _key = int.MinValue;
        _val = int.MinValue;
        _next = null;
    }

    public KeyNode(int key, int val): this() {
        _key = key;
        _val = val;
    }
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache obj = new LRUCache(capacity);
 * int param_1 = obj.Get(key);
 * obj.Put(key,value);
}

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