leetcode 146. LRU Cache

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

其实最主要还是要单独写一个DListNode 的class出来，然后把add和remove方法都单独写好。

注意用了key-node这样一个哈希表，便于快速将argument里面的key定位到相应的含有键值对的node上。其实这个套路很像61B的pj3.

class LRUCache {
    class DListNode {
        int key;
        int value;
        DListNode prev;
        DListNode next;
    }
    
    public void addNode(DListNode node) {
        node.prev = head;
        node.next = head.next;
        head.next.prev = node;
        head.next = node;
    }
    public void removeNode(DListNode node) {
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }
    public void moveToHead(DListNode node) {
        this.removeNode(node);
        this.addNode(node);
    }
    
    private Map<Integer,DListNode> ht = new HashMap<Integer,DListNode>();
    int c = 0;
    int capacity;
    DListNode head,tail;
    
    public LRUCache(int capacity) {
        this.capacity = capacity;
        head = new DListNode();
        tail = new DListNode();
        head.next = tail;
        head.prev = null;
        tail.prev = head;
        tail.next = null;  
    }
    
    public int get(int key) {
        DListNode node  = ht.get(key);
        if(node == null) {
            return -1;
        }
        this.moveToHead(node);
        return node.value;
        
    }
    
    public void put(int key, int value) {
        DListNode node = ht.get(key);
        if(node == null) {
            DListNode n = new DListNode();
            n.key = key;
            n.value = value;
            this.addNode(n);
            ht.put(key,n);
            c++;
            if(c>capacity) {
                ht.remove(tail.prev.key);
                this.removeNode(tail.prev);
                c--;
            }
        }else {
            node.value = value;
            this.moveToHead(node);
        }
    }
}