146. LRU Cache

必会题目！！！
Design and implement a data structure for Least Recently Used (LRU) cache. 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.

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

java

class Node {    Node next;    Node prev;    int key;    int value;    public Node(int key, int value) {        this.key = key;        this.value = value;        this.next = null;        this.prev = null;    }}public class LRUCache {    int capacity;    Map<Integer, Node> map = new HashMap<>();    Node head = new Node(-1, -1);    Node tail = new Node(-1, -1);    public LRUCache(int capacity) {        this.capacity = capacity;        head.next = tail;        tail.prev = head;    }    public int get(int key) {        int val = 0;        if (map.containsKey(key)) {            Node curr = map.get(key);            curr.prev.next = curr.next;            curr.next.prev = curr.prev;            val = map.get(key).value;            moveTail(map.get(key));        } else {            val = -1;        }        return val;    }    public void put(int key, int value) {        if (get(key) != -1) {            map.get(key).value = value;            return;        }         if(map.size() == capacity) {            map.remove(head.next.key);            moveHead();        }         map.put(key, new Node(key, value));        moveTail(map.get(key));    }    private void moveTail(Node node) {        Node left = tail.prev;        left.next = node;        node.prev = left;        node.next = tail;        tail.prev = node;    }    private void moveHead() {        head.next = head.next.next;        head.next.prev = head;    }}/** * 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); */