LRUCache.java

/*
 Author:     Andy, nkuwjg@gmail.com
 Date:       March 12, 2014
 Problem:    LRU Cache
 Difficulty: Hard
 Source:     http://oj.leetcode.com/problems/lru-cache/
 Notes:
 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: Hash + list.
*/

import java.util.LinkedHashMap;
import java.util.Map;
public class LRUCache {
    private Map<Integer, Integer> map;
    private int capacity;
    public LRUCache(int capacity) {
        this.capacity = capacity;
        map = new LinkedHashMap<Integer, Integer>(capacity + 1);
    }
    
    public int get(int key) {
        Integer val = map.get(key);
        if (val == null) return -1;
        map.remove(key);
        map.put(key, val);
        return val;
    }
    
    public void set(int key, int value) {
        map.remove(key);
        map.put(key, value);
        if (map.size() > capacity)
            map.remove(map.entrySet().iterator().next().getKey());
    }
}