题目：
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.
链接： http://leetcode.com/problems/lru-cache/
5/14/2017
准备面试
用hashmap + double linkedlist实现，hashmap里key是key, value是double linkedlist里面的node节点
注意的问题：
1. 最好不用系统的Deque来做double linkedlist，不容易储存node
2. double linkedlist里的node包括key, value, prev, next
3. 可以不需要额外的size而直接用map的size和capacity做比较
4. head, tail可以设成dummy node，简化最开始的判断
5. put包含get，可以省略一些步骤
6. 第31，32行是将旧的值所在的node从double linkedlist当中取出来，相当于remove出来，之后往里加的时候只需要按照一般的情况来就可以了。
7. 第42行可以直接在map.get(key).val = value来更改node的值，当然node的位置在double linkedlist当中需要更改。
1 public class LRUCache { 2 class Node { 3 int key; 4 int val; 5 Node prev; 6 Node next; 7 Node(int key, int val) { 8 this.key = key; 9 this.val = val; 10 } 11 } 12 Map<Integer, Node> map; 13 Node head; 14 Node tail; 15 int capacity; 16 17 public LRUCache(int capacity) { 18 this.capacity = capacity; 19 this.map = new HashMap<Integer, Node>(); 20 this.head = new Node(-1, -1); 21 this.tail = new Node(-1, -1); 22 // initialize head / tail 23 tail.prev = head; 24 head.next = tail; 25 } 26 27 public int get(int key) { 28 if (map.containsKey(key)) { 29 Node old = map.get(key); 30 // why need these two? treat old node as a new node, similar idea as put remove last node 31 old.prev.next = old.next; 32 old.next.prev = old.prev; 33 moveToTail(old); 34 return old.val; 35 } else { 36 return -1; 37 } 38 } 39 40 public void put(int key, int value) { 41 if (get(key) != -1) { 42 map.get(key).val = value; 43 } else { 44 // check capacity first 45 if (map.size() == this.capacity) { 46 // remove last node from map 47 map.remove(head.next.key); 48 head.next = head.next.next; 49 head.next.prev = head; 50 } 51 Node node = new Node(key, value); 52 moveToTail(node); 53 map.put(key, node); 54 } 55 } 56 private void moveToTail(Node node) { 57 node.prev = tail.prev; 58 tail.prev = node; 59 node.prev.next = node; 60 node.next = tail; 61 } 62 } 63 64 /** 65 * Your LRUCache object will be instantiated and called as such: 66 * LRUCache obj = new LRUCache(capacity); 67 * int param_1 = obj.get(key); 68 * obj.put(key,value); 69 */
linkedhashmap
https://discuss.leetcode.com/topic/17433/probably-the-best-java-solution-extend-linkedhashmap
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