[Java] 利用LinkedHashMap来实现LRU Cache

http://www.acmerblog.com/leetcode-lru-cache-lru-5745.html

https://oj.leetcode.com/discuss/1188/java-is-linkedhashmap-considered-cheating

http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashMap.html#removeEldestEntry(java.util.Map.Entry)