- Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and put.
- get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
- put(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.
The cache is initialized with a positive capacity.
- Follow up:
- Could you do both operations in O(1) time complexity?
- Example:
- LRUCache cache = new LRUCache( 2 /* capacity */ );
- cache.put(1, 1);
- cache.put(2, 2);
- cache.get(1); // returns 1
- cache.put(3, 3); // evicts key 2
- cache.get(2); // returns -1 (not found)
- cache.put(4, 4); // evicts key 1
- cache.get(1); // returns -1 (not found)
- cache.get(3); // returns 3
- cache.get(4); // returns 4
题目大意:
实现一个 LRU 结构.
解法:
采用 Hashmap + 双向链表进行实现, 双向链表记录访问次序, HashMap 帮助实现时间复杂度为 O(1) 的查找效率.
java:
- class LRUCache {
- private int capacity;
- private int count;
- private DLinkedNode head;
- private DLinkedNode tail;
- private Map<Integer,DLinkedNode>cache;
- class DLinkedNode{
- int key;
- int value;
- DLinkedNode pre;
- DLinkedNode next;
- public DLinkedNode(){ }
- public DLinkedNode(int key,int value){
- this.key=key;
- this.value=value;
- this.pre=null;
- this.next=null;
- }
- }
- public LRUCache(int capacity) {
- this.count=0;
- this.capacity=capacity;
- this.cache=new HashMap<>();
- head=new DLinkedNode();
- tail=new DLinkedNode();
- head.pre=null;
- tail.next=null;
- head.next=tail;
- tail.pre=head;
- }
- private void addTohead(DLinkedNode node){
- node.next=head.next;
- head.next.pre=node;
- node.pre=head;
- head.next=node;
- }
- private void removeNode(DLinkedNode node){
- node.next.pre=node.pre;
- node.pre.next=node.next;
- node.pre=null;
- node.next=null;
- }
- private void moveToHead(DLinkedNode node){
- removeNode(node);
- addTohead(node);
- }
- private void popTail(){
- DLinkedNode tmp=tail.pre.pre;
- tmp.next=tail;
- tail.pre=tmp;
- }
- public int get(int key) {
- DLinkedNode tmp=cache.get(key);
- if (tmp!=null){
- moveToHead(tmp);
- return tmp.value;
- }
- return -1;
- }
- public void put(int key, int value) {
- if(!cache.containsKey(key)){
- DLinkedNode node=new DLinkedNode(key,value);
- addTohead(node);
- cache.put(key,node);
- count++;
- if (count>capacity){
- cache.remove(tail.pre.key);
- popTail();
- count--;
- }
- }else{
- DLinkedNode tmp=cache.get(key);
- tmp.value=value;
- moveToHead(tmp);
- }
- }
- }
来源: http://www.bubuko.com/infodetail-3108675.html