LRU 缓存机制

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mb5fed6f751964c 2021-10-13 23:08:00

运用你所掌握的数据结构，设计和实现一个 LRU (最近最少使用) 缓存机制。

实现 LRUCache 类：

LRUCache(int capacity) 以正整数作为容量 capacity 初始化 LRU 缓存

int get(int key) 如果关键字 key 存在于缓存中，则返回关键字的值，否则返回 -1 。

void put(int key, int value) 如果关键字已经存在，则变更其数据值；如果关键字不存在，则插入该组「关键字-值」。当缓存容量达到上限时，它应该在写入新数据之前删除最久未使用的数据值，从而为新的数据值留出空间。

import java.util.HashMap;
import java.util.Map;

class LRUCache {

private int capacity;

private Map<Integer, Node> indexMap;

private DoubleList doubleList;

static class Node {
private int key;
private int value;
private Node pre;
private Node next;

public Node(int key, int value) {
this.key = key;
this.value = value;
        }
    }

static class DoubleList {
private Node head;
private Node tail;

private void add(Node node) {
if (this.head == null) {
this.head = node;
this.tail = this.head;
            } else {
this.tail.next = node;
node.pre = this.tail;
this.tail = node;
            }
        }

private Node removeOldest() {
if (this.head == null) {
throw new NullPointerException();
            }
Node removed = this.head;
if (this.head == this.tail) {
this.head = null;
this.tail = null;
            } else {
this.head.next.pre = null;
this.head = this.head.next;
            }
return removed;
        }

private void moveToTail(Node node) {
if (node == null) {
return;
            }
if (this.tail == node) {
return;
            }
if (this.head == node) {
removeOldest();
add(node);
return;
            }
node.pre.next = node.next;
node.next.pre = node.pre;
add(node);
        }
    }

public LRUCache(int capacity) {
this.capacity = capacity;
this.indexMap = new HashMap<>();
this.doubleList = new DoubleList();
    }

public int get(int key) {
Node node = this.indexMap.get(key);
if (node == null) {
return -1;
        }
doubleList.moveToTail(node);
return node.value;
    }

public void put(int key, int value) {
Node node = this.indexMap.get(key);
if (node != null) {
node.value = value;
this.doubleList.moveToTail(node);
        } else {
if (this.indexMap.size() == this.capacity) {
Node removeOldest = doubleList.removeOldest();
this.indexMap.remove(removeOldest.key);
            }
node = new Node(key, value);
this.doubleList.add(node);
this.indexMap.put(key, node);
        }
    }
}