LeetCode刷题笔录LRUCache

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.

set(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.

如果要保证O(1)的读取，那么肯定是要用hashmap来存储键值对的；同时又要维护一个队列，并保证插入队首和删除一个node都是O(1)，则需要一个double linked list。因此建立一个Node类作为双链表的node，存储key, value, prev和next.

public class LRUCache {    private Map<Integer, Node> map;    private Node head;    private Node tail;    private int capacity;    private int count;    public LRUCache(int capacity) {        map = new HashMap<Integer, Node>();        this.capacity = capacity;        head = null;        tail = null;        count = 0;    }        private void setHead(Node node){        node.next = head;        node.prev = null;        if(head != null){            head.prev = node;        }        head = node;        if(tail == null){            tail = head;        }    }        private void deleteNode(Node node){        if(node.prev != null){            node.prev.next = node.next;        }        else{            head = node.next;        }        if(node.next != null){            node.next.prev = node.prev;        }        else{            tail = node.prev;        }    }    public int get(int key) {        if(map.containsKey(key)){            Node node = map.get(key);            deleteNode(node);            setHead(node);            return node.value;        }                    else            return -1;    }        public void set(int key, int value) {        if(map.containsKey(key)){            Node node = map.get(key);            node.value = value;            deleteNode(node);            setHead(node);        }        else{            Node node = new Node(key, value);            if(count < capacity){                map.put(key, node);                setHead(node);                count++;            }            else{                //deleteNode(tail);                map.remove(tail.key);                tail = tail.prev;                if(tail != null)                    tail.next = null;                map.put(key, node);                setHead(node);            }        }    }            class Node{        int key;        int value;        Node prev;        Node next;        public Node(int key, int value){            this.key = key;            this.value = value;            prev = null;            next = null;        }    }}