Leetcode: LRU Cache 解题报告

浏览数：71 / 时间：2015年06月20日

LRU Cache

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.

SOLUTION 1:

利用了JAVA 自带的LinkedHashMap ，其实这相当于作弊了，面试官不太可能会过。但是我们仍然可以练习一下如何使用LinkedHashMap.

同学们可以参考一下它的官方文档：

https://docs.oracle.com/javase/7/docs/api/java/util/LinkedHashMap.html#LinkedHashMap(int)

1. OverRide removeEldestEntry 函数，在Size达到最大值最，删除最长时间未访问的节点。

protected boolean removeEldestEntry(Map.Entry eldest) {

return size() > capacity;

}

2. 在Get/ Set的时候，都更新节点，即删除之，再添加之，这样它会作为最新的节点加到双向链表中。

 1 package Algorithms.hash;
 2 
 3 import java.util.LinkedHashMap;
 4 import java.util.Map;
 5 
 6 public class LRUCache2 {
 7     public static void main(String[] strs) {
 8         LRUCache2 lrc2 = new LRUCache2(2);
 9         lrc2.set(1,3);
10         lrc2.set(2,2);
11         lrc2.set(1,4);
12         lrc2.set(4,2);
13         
14         System.out.println(lrc2.get(1));
15     }
16     
17     LinkedHashMap<Integer, Integer> map;
18     int capacity;
19     
20     public LRUCache2(final int capacity) {
21         // create a map.
22         map = new LinkedHashMap<Integer, Integer>(capacity) {
23             /**
24              * 
25              */
26             private static final long serialVersionUID = 1L;
27 
28             protected boolean removeEldestEntry(Map.Entry eldest) {
29                 return size() > capacity;
30             }
31         };
32         this.capacity = capacity;
33     }
34     
35     public int get(int key) {
36         Integer ret = map.get(key);
37         if (ret == null) {
38             return -1;
39         } else {
40             map.remove(key);
41             map.put(key, ret);
42         }
43         
44         return ret;
45     }
46     
47     public void set(int key, int value) {
48         map.remove(key);
49         map.put(key, value);
50     }
51 }