标签：des style blog color io os 使用 java strong
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.
LRU cache, 采用“如果满则淘汰最近未使用的项目“的策略，所以需要一个序列，每次插入的项目放入序列首，被查找的项目也移至序列首。如果超过数量上限则删除序列最后的项目。查找，插入操作都需要O(1)的时间复杂度，否则超时通不过。所以这个序列可以是链表，并且为了查找快捷，需要结合HashMap。由于涉及到转移项目操作，需要双向链表。
HashMap用于把查找键key和具体项目联系起来，这样就能直接获得项目，然后将其移至链表首位。
需要注意的是set操作中，如果set的key已经存在了，那就相当于一次查找操作加上修改其值和HashMap的操作。
代码如下：
1 class Node 2 { 3 int key; 4 int value; 5 Node next; 6 Node front; 7 Node(int k, int v) 8 { 9 key=k; 10 value=v; 11 next=null; 12 front=null; 13 } 14 } 15 16 private HashMap<Integer, Node> hs; 17 private Node head; 18 private Node tail; 19 private int cap; 20 public LRUCache(int capacity) { 21 hs = new HashMap<Integer, Node>(); 22 head = null; 23 cap = capacity; 24 tail = null; 25 } 26 27 public int get(int key) { 28 Node re = hs.get(key); 29 if(re==null) 30 return -1; 31 if(re.front==null) 32 return re.value; 33 re.front.next = re.next; 34 if(re.next!=null) 35 re.next.front = re.front; 36 else 37 tail = re.front; 38 re.front = null; 39 re.next = head; 40 head.front = re; 41 head = re; 42 return re.value; 43 } 44 45 public void set(int key, int value) { 46 Node temp = hs.get(key); 47 if(temp!=null) 48 { 49 get(key); 50 head.value = value; 51 hs.put(key,head); 52 } 53 else 54 { 55 temp = new Node(key, value); 56 temp.next = head; 57 if(head!=null) 58 head.front = temp; 59 head = temp; 60 if(hs.size()==0) 61 tail = temp; 62 hs.put(key, temp); 63 if(hs.size()>cap) 64 { 65 hs.remove(tail.key); 66 tail.front.next = null; 67 tail = tail.front; 68 } 69 } 70 }
标签：des style blog color io os 使用 java strong
原文地址：http://www.cnblogs.com/splash/p/4002959.html
