Updated LRU solution

Author: RodneyShag

Chp. 16 - More Problems (Moderate)/_16_25_LRU_Cache/DoublyLinkedList.java

@@ -1,40 +1,22 @@
 package _16_25_LRU_Cache;
 
 public class DoublyLinkedList {
- Node head = null;
- Node tail = null;
+ private Node head = null;
+ private Node tail = null;
 
- void addFirst(Node n) {
+ public void addFirst(Node n) {
  if (head == null) {
  head = n;
  tail = n;
  } else {
+ n.prev = null;
  n.next = head;
  head.prev = n;
  head = n;
  }
  }
- 
- void removeLast() {
- removeNode(tail);
-// if (tail == null) {
-// return null;
-// } 
-// if (head == tail) { // only 1 Node in List
-// Node n = tail;
-// head = null;
-// tail = null;
-// return n;
-// } else {
-// Node n = tail;
-// tail = n.prev;
-// tail.next = null;
-// n.prev = null;
-// return n;
-// }
- }
- 
- void removeNode(Node n) {
+
+ public void remove(Node n) { // Assumes "Node n" is in this list
  if (n == null) {
  return;
 	}
@@ -51,4 +33,17 @@ void removeNode(Node n) {
  tail = n.prev;
 	}
  }
+ 
+ public void updateFreshness(Node n) { // Assumes "Node n" is in this list
+ remove(n);
+ addFirst(n);
+ }
+
+ public Node getHead() {
+ return head;
+ }
+
+ public Node getTail() {
+ return tail;
+ }
 }

Chp. 16 - More Problems (Moderate)/_16_25_LRU_Cache/LRUCache.java

@@ -11,9 +11,9 @@
 // Answer: Because when we remove from end of Linked List, the Node's key is used
 // to find the Node in the HashMap (to remove the node there as well)
 
-class LRUCache {
+public class LRUCache {
  private int maxSize;
- private Map<Integer, Node> map; // gives us constant access to Nodes
+ private Map<Integer, Node> map; // gives us O(1)-time access to Nodes
  private DoublyLinkedList dll; // used to keep track of "freshness" of Nodes
 
  public LRUCache(int maxSize) {
@@ -22,36 +22,34 @@ public LRUCache(int maxSize) {
  dll = new DoublyLinkedList();
  }
 
- public void add(Integer key, Node value) {
- if (map.containsKey(key)) {
- get(key);
- } else {
- if (map.size() == maxSize) {
- Node n = dll.tail;
- dll.removeLast();
- map.remove(n.key);
- }
- map.put(key, value);
- dll.addFirst(value);
+ public void add(int key, String value) {
+ remove(key); // If key already exists, we will overwrite it.
+ if (map.size() == maxSize) {
+ remove(dll.getTail().key);
  }
+ Node n = new Node(key, value);
+ dll.addFirst(n);
+ map.put(key, n);
  }
 
- public Node get(Integer key) {
+ public void remove(int key) {
  Node n = map.get(key);
+ dll.remove(n);
+ map.remove(key);
+ }
 
- // Update Freshness
- if (n.prev != null) {
- n.prev.next = n.next;
- }
- if (n.next != null) {
- n.next.prev = n.prev;
- }
- dll.addFirst(n);
-
- return n;
+ public String getValue(int key) {
+ Node n = map.get(key);
+ if (n == null) {
+ return null;
+	}
+ if (n != dll.getHead()) {
+ dll.updateFreshness(n);
+	}
+ return n.value;
  }
 
- public DoublyLinkedList getItems() {
+ public DoublyLinkedList getItems() { // added for testing
  return dll;
  }
 }

Chp. 16 - More Problems (Moderate)/_16_25_LRU_Cache/Node.java

@@ -1,7 +1,7 @@
 package _16_25_LRU_Cache;
 
 public class Node {
-int key;
+ int key;
  String value;
  Node next;
  Node prev;
@@ -12,7 +12,7 @@ public Node(int k, String v) {
  next = null;
  prev = null;
  }
- 
+
  @Override
  public String toString() {
  return value;

Chp. 16 - More Problems (Moderate)/_16_25_LRU_Cache/Tester.java

@@ -7,15 +7,13 @@ public static void main(String[] args) {
  /* Create LRU Cache */
  LRUCache cache = new LRUCache(7);
  for (int i = 20; i >= 1; i--) {
- Node node = new Node(i, "Node " + i + " value");
- cache.add(i, node);
+ cache.add(i, "Node " + i + " value");
  }
 
  /* Access Item 4, and print cache items */
- cache.get(4); // should move it to head of cache
- DoublyLinkedList items = cache.getItems();
- Node head = items.head;
- Node n = head;
+ cache.remove(1);
+ cache.getValue(4); // should move it to head of cache
+ Node n = cache.getItems().getHead();
  while (n != null) {
  System.out.println(n);
  n = n.next;