leetcode146- LRU Cache- hard

Posted 2020-10-16 jasminemzy

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

 

数据结构：双端链表(自己实现有prev和next的Node) + Map<Key, Node>。Node具体包含(prev, next, key, val)。map具体包含(key值，指过去对应的node)。辅助的全局变量：Node head, tail，记得一开始先让它们两个连上。

辅助的小函数：1.把节点加到最前面。(在创造新节点、更新状态的时候都用到) 2.把某key对应的那个节点拉到最前面（更新的时候用到）。 3.删除最后一个节点（满了以后更新的时候用到）

get：1.更新状态。 2.返回值。

put：三种情况

a)已经有了的话

　　1.改值。2.更新状态。

b)没有但还有位置

　　1.造节点。 2.map加映射。 3.插到最前面。

c)没有而且没位置了

　　1.map删最后的节点。2.list删最后的节点。 3.造结点。4.map加映射。5.插到最前面。

 

实现： 

 

class LRUCache {
    
    private int capacity;
    private Map<Integer, Node> map;
    private Node head;
    private Node tail;
    
    private class Node {
        public int key;
        public int val;
        public Node prev;
        public Node next;
        public Node(int key, int val) {
            this.key = key;
            this.val = val;
            this.prev = null;
            this.next = null;
        }
    }

    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.map = new HashMap<Integer, Node>();
        this.head = new Node(-1, -1);
        this.tail = new Node(-1, -1);
        this.head.next = this.tail;
        this.tail.prev = this.head;
    }

    public int get(int key) {
        if (!map.containsKey(key)) {
            return -1;
        }
        update(key);
        return map.get(key).val;
    }

    public void put(int key, int value) {
        if (map.containsKey(key)) {
            map.get(key).val = value;
            update(key);
        } else if (map.size() < capacity) {
            Node node = new Node(key, value);
            map.put(key, node);
            addToHead(node);
        } else {
            Node lastNode = tail.prev;
            map.remove(lastNode.key);
            lastNode.prev.next = tail;
            tail.prev = lastNode.prev;
            Node node = new Node(key, value);
            map.put(key, node);
            addToHead(node);
        }
        return;
    }

    private void update(int key) {
        Node node = map.get(key);
        node.prev.next = node.next;
        node.next.prev = node.prev;
        addToHead(node);
    }

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

/**
 * 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);
 */