【leetcode刷题笔记】LRU Cache

 1 public class LRUCache {
 2     private class Node{
 3         int value;
 4         int key;
 5         Node before;
 6         Node after;
 7         public Node(int key,int value){
 8             this.value = value;
 9             this.key = key;
10             before = null;
11             after = null;
12         }
13     }
14     
15     private int capacity;
16     private HashMap<Integer, Node> map = new HashMap<Integer,Node>();
17     private Node headNode = new Node(-1, -1);
18     private Node tailNode = new Node(-1, -1);
19     
20     public LRUCache(int capacity) {
21         this.capacity = capacity;
22         headNode.after = tailNode;
23         tailNode.before = headNode;
24     }
25     
26     public void move_to_head(Node current){
27         current.after = headNode.after;
28         headNode.after = current;
29         current.before = headNode;
30         current.after.before = current;
31     }
32     public int get(int key) {
33         //if we don‘t have this node
34         if(!map.containsKey(key))
35             return -1;
36         
37         //if we have this node, get it and move it to head
38         Node current = map.get(key);
39         current.before.after = current.after;
40         current.after.before = current.before;
41         move_to_head(current);
42         
43         return map.get(key).value;        
44     }
45     
46     public void set(int key, int value) {
47         //if we already have this node,just change its value
48         if(get(key) != -1){
49             map.get(key).value = value;
50             return;
51         }
52         
53         //if we indeed don‘t have this node, we first check capacity
54         if(map.size() == capacity){
55             map.remove(tailNode.before.key);
56             tailNode.before.before.after = tailNode;
57             tailNode.before = tailNode.before.before;
58         }
59         
60         //now we are sure we have space for this new node,put it ahead of the list
61         Node newNode = new Node(key, value);
62         map.put(key, newNode);
63         move_to_head(newNode);
64     }
65 }