Class LRUCache

146 LRU Cache/146 LRU Cache.java:12–77

import java.util.HashMap;

public class LRUCache {
    private static class Node {
        int key;
        int value;
        Node prev;
        Node next;

        public Node(int key, int value) {
            this.key = key;
            this.value = value;
        }
    }

    private final int capacity;
    private final HashMap<Integer, Node> map;
    private final Node head;
    private final Node tail;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        map = new HashMap<>();
        head = new Node(0, 0);
        tail = new Node(0, 0);
        head.next = tail;
        tail.prev = head;
    }

    public int get(int key) {
        if (!map.containsKey(key)) {
            return -1;
        }
        Node node = map.get(key);
        remove(node);
        insertAtHead(node);
        return node.value;
    }

    public void put(int key, int value) {
        if (map.containsKey(key)) {
            Node node = map.get(key);
            node.value = value;
            remove(node);
            insertAtHead(node);
        } else {
            if (map.size() == capacity) {
                map.remove(tail.prev.key);
                remove(tail.prev);
            }
            Node newNode = new Node(key, value);
            map.put(key, newNode);
            insertAtHead(newNode);
        }
    }

    private void remove(Node node) {
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }