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0146. LRU Cache
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‎markdown/0146. LRU Cache.md

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### [146\. LRU Cache](https://leetcode.com/problems/lru-cache/)
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> 我使用双向循环链表和 hashtable 完成了，这里有一个小细节就是 put 的时候也是访问，需要将对应的 Node 提取到 head 上。
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Difficulty: **Medium**
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Design and implement a data structure for . It should support the following operations: `get` and `put`.
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`get(key)` - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
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`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.
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The cache is initialized with a **positive** capacity.
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**Follow up:**
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Could you do both operations in **O(1)** time complexity?
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**Example:**
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```
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LRUCache cache = new LRUCache( 2 /* capacity */ );
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cache.put(1, 1);
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cache.put(2, 2);
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cache.get(1); // returns 1
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cache.put(3, 3); // evicts key 2
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cache.get(2); // returns -1 (not found)
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cache.put(4, 4); // evicts key 1
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cache.get(1); // returns -1 (not found)
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cache.get(3); // returns 3
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cache.get(4); // returns 4
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```
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#### Solution
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Language: **Java**
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```java
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class LRUCache {
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class LRUValue {
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int key;
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int value;
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LRUValue next; //链表下一个节点的key
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LRUValue pre;// 链表上一个节点的key
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public LRUValue(int key, int value, LRUValue next, LRUValue pre) {
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this.key = key;
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this.value = value;
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this.next = next;
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this.pre = pre;
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}
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}
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private Map<Integer, LRUValue> map = new HashMap<>();
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private int capacity;
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private LRUValue head;
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public LRUCache(int capacity) {
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this.capacity = capacity;
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}
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public int get(int key) {
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if (!map.containsKey(key)) {
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return -1;
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} else {
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LRUValue lruValue = map.get(key);
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if (lruValue != head) { // 如果获取的元素不是在头部，需要将该元素升级到头部
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promotionNode(lruValue);
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}
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return head.value;
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}
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}
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public void put(int key, int value) {
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if (map.size() == 0) {
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LRUValue lruValue = new LRUValue(key, value, null, null);
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lruValue.next = lruValue;
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lruValue.pre = lruValue;
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head = lruValue;
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map.put(key, lruValue);
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} else {
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LRUValue targetValue = map.get(key);
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if (targetValue != null) {// 注意 put 也算是访问，需要把它提到 header
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if (targetValue != head) {
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promotionNode(targetValue);
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}
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targetValue.value = value;
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} else {
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if (map.size() >= capacity) {
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int removeKey = head.pre.key;
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head.pre = head.pre.pre;
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map.remove(removeKey);
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}
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LRUValue lruValue = new LRUValue(key, value, head, head.pre);
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map.put(key, lruValue);
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head.pre.next = lruValue;
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head.pre = lruValue;
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head = lruValue;
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}
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}
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}
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private void promotionNode(LRUValue lruValue) {
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lruValue.pre.next = lruValue.next;
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lruValue.next.pre = lruValue.pre;
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head.pre.next = lruValue;
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lruValue.next = head;
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lruValue.pre = head.pre;
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head.pre = lruValue;
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head = lruValue;
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}
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}
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/**
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* Your LRUCache object will be instantiated and called as such:
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* LRUCache obj = new LRUCache(capacity);
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* int param_1 = obj.get(key);
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* obj.put(key,value);
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*/
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```
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![](https://raw.githubusercontent.com/PicGoBed/PicBed/master/20190724010539.png)

‎src/main/java/leetcode/_146_/LRUCache.java

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package leetcode._146_;
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import java.util.HashMap;
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import java.util.Map;
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/**
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* Created by zhangbo54 on 2019-07-23.
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*/
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class LRUCache {
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class LRUValue {
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int key;
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int value;
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LRUValue next; //链表下一个节点的key
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LRUValue pre;// 链表上一个节点的key
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public LRUValue(int key, int value, LRUValue next, LRUValue pre) {
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this.key = key;
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this.value = value;
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this.next = next;
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this.pre = pre;
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}
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}
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private Map<Integer, LRUValue> map = new HashMap<>();
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private int capacity;
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private LRUValue head;
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public LRUCache(int capacity) {
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this.capacity = capacity;
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}
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public int get(int key) {
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if (!map.containsKey(key)) {
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return -1;
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} else {
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LRUValue lruValue = map.get(key);
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if (lruValue != head) { // 如果获取的元素不是在头部，需要将该元素升级到头部
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promotionNode(lruValue);
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}
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return head.value;
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}
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}
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public void put(int key, int value) {
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if (map.size() == 0) {
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LRUValue lruValue = new LRUValue(key, value, null, null);
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lruValue.next = lruValue;
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lruValue.pre = lruValue;
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head = lruValue;
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map.put(key, lruValue);
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} else {
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LRUValue targetValue = map.get(key);
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if (targetValue != null) {// 注意 put 也算是访问，需要把它提到 header
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if (targetValue != head) {
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promotionNode(targetValue);
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}
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targetValue.value = value;
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} else {
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if (map.size() >= capacity) {
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int removeKey = head.pre.key;
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head.pre = head.pre.pre;
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map.remove(removeKey);
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}
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LRUValue lruValue = new LRUValue(key, value, head, head.pre);
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map.put(key, lruValue);
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head.pre.next = lruValue;
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head.pre = lruValue;
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head = lruValue;
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}
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}
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}
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private void promotionNode(LRUValue lruValue) {
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lruValue.pre.next = lruValue.next;
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lruValue.next.pre = lruValue.pre;
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head.pre.next = lruValue;
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lruValue.next = head;
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lruValue.pre = head.pre;
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head.pre = lruValue;
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head = lruValue;
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}
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}
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/**
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* Your LRUCache object will be instantiated and called as such:
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* LRUCache obj = new LRUCache(capacity);
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* int param_1 = obj.get(key);
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* obj.put(key,value);
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*/

‎src/main/java/leetcode/_146_/Main.java

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package leetcode._146_;
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/**
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* Created by zhangbo54 on 2019-03-04.
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*/
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public class Main {
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public static void main(String[] args) {
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// LRUCache lruCache = new LRUCache(2);
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// lruCache.put(1, 1);
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// lruCache.put(2, 2);
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// System.out.println(lruCache.get(1));
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// lruCache.put(3, 3);
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// System.out.println(lruCache.get(2));
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// lruCache.put(4, 4);
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// System.out.println(lruCache.get(1));
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// System.out.println(lruCache.get(3));
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// System.out.println(lruCache.get(4));
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//
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//
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// System.out.println("------------");
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//
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// LRUCache lruCache2 = new LRUCache(2);
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// lruCache2.put(2, 1);
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// lruCache2.put(1, 1);
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// lruCache2.put(2, 3);
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// lruCache2.put(4, 1);
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// System.out.println(lruCache2.get(1));
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// System.out.println(lruCache2.get(2));
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//
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// System.out.println("------------");
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//
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// LRUCache lruCache3 = new LRUCache(2);
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// System.out.println(lruCache3.get(2));
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// lruCache3.put(2, 6);
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// System.out.println(lruCache3.get(1));
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// lruCache3.put(1, 5);
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// lruCache3.put(1, 2);
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// System.out.println(lruCache3.get(1));
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// System.out.println(lruCache3.get(2));
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//
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// System.out.println("------------");
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LRUCache lruCache4 = new LRUCache(3);
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lruCache4.put(1, 1);
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lruCache4.put(2, 2);
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lruCache4.put(3, 3);
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lruCache4.put(4, 4);
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System.out.println(lruCache4.get(4));
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System.out.println(lruCache4.get(3));
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System.out.println(lruCache4.get(2));
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System.out.println(lruCache4.get(1));
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lruCache4.put(5, 5);
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System.out.println(lruCache4.get(1));
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System.out.println(lruCache4.get(2));
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System.out.println(lruCache4.get(3));
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System.out.println(lruCache4.get(4));
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System.out.println(lruCache4.get(5));
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}
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}
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