LeetCode 146. LRU Cache

146. LRU Cache

Difficulty: Medium

Design and implement a data structure for . It should support the following operations: get and put.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
put(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.

The cache is initialized with a positive capacity.

Follow up:
Could you do both operations in O(1) time complexity?

Example:

LRUCache cache = new LRUCache( 2 /* capacity */ );

cache.put(1, 1);
cache.put(2, 2);
cache.get(1);       // returns 1
cache.put(3, 3);    // evicts key 2
cache.get(2);       // returns -1 (not found)
cache.put(4, 4);    // evicts key 1
cache.get(1);       // returns -1 (not found)
cache.get(3);       // returns 3
cache.get(4);       // returns 4

Solution

Language: Java

class LRUCache {

    class Node {
        int key;
        int val;
        Node next;
        Node prev;
        public Node(int key, int val) {
            this.key = key;
            this.val = val;
        }
        
        public void insertNext(Node n) {
            if (next == null) {
                next = n;
                n.prev = this;
            } else {
                n.next = next;
                next.prev = n;
                next = n;
                n.prev = this;
            }
        }
        
        public void removeNext() {
            if (next == null) {
                return;
            }
            if (next.next == null) {
                next.prev = null;
                next = null;
                return;
            }
            Node nn = next.next;
            next.prev = null;
            next.next = null;
            next = nn;
            nn.prev = this;
        }
    }
    
    Node dummy;
    Node tail;
    Map<Integer, Node> map;
    int size;
    int cap;
    
    public LRUCache(int capacity) {
        cap = capacity;
        size = 0;
        dummy = new Node(0, 0);
        tail = new Node(0, 0);
        tail.prev = dummy;
        dummy.next = tail;
        map = new HashMap<>();
    }
    
    public int get(int key) {
        if (!map.containsKey(key)) {
            return -1;
        } else {
            int val = map.get(key).val;
            put(key, val);
            return val;
        }
    }
    
    public void put(int key, int value) {
        if (map.containsKey(key)) {
            Node n = map.get(key);
            n.val = value;
            n.prev.removeNext();
            dummy.insertNext(n);
        } else {
            if (size >= cap) {
                Node n = tail.prev;
                map.remove(n.key);
                tail.prev.prev.removeNext();
            } else {
                size++;
            }
            dummy.insertNext(new Node(key, value));
            map.put(key, dummy.next);
        }
    }
}

/**
 * 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);
 */