Merge pull request #283 from noebrito/master

RecentlyUsed Java problem

Java/RecentlyUsed.java

@@ -0,0 +1,70 @@
+import java.util.Map;
+
+/**
+ * Maintain URLs visited in most recently visited order.
+ */
+public class RecentlyUsed {
+
+    Map<String, ListNode> visitedURLs = new HashMap<>();
+    ListNode dummyHead = new ListNode("dummy");
+
+    public void addUrl(String url) {
+
+        if (visitedURLs.containsKey(url)) {
+            ListNode node = visitedURLs.get(url);
+            ListNode prev = node.prev;
+            ListNode next = node.next;
+
+            prev.next = next;
+            next.prev = prev;
+            addToHead(node);
+
+        } else {
+            ListNode newNode = new ListNode(url);
+            visitedURLs.put(url, newNode);
+            addToHead(newNode);
+        }
+
+    }
+
+    public void printVisitedUrls() {
+        ListNode node = dummyHead.next;
+        while (node != null) {
+            System.out.println(node.url);
+            node = node.next;
+        }
+    }
+
+    private void addToHead(ListNode newNode) {
+        ListNode next = dummyHead.next;
+        dummyHead.next = newNode;
+        newNode.prev = dummyHead;
+        newNode.next = next;
+        if (next != null) {
+            next.prev = newNode;
+        }
+    }
+
+    public static void main(String[] args) {
+        RecentlyUsed recentlyUsed = new RecentlyUsed();
+        recentlyUsed.addUrl("www.facebook.com");
+        recentlyUsed.addUrl("www.google.com");
+        recentlyUsed.addUrl("www.apple.com");
+        System.out.println("Should print in order: apple, google, facebook");
+        recentlyUsed.printVisitedUrls();
+        recentlyUsed.addUrl("www.google.com");
+        System.out.println("Should print in order: google, apple, facebook");
+        recentlyUsed.printVisitedUrls();
+    }
+
+    static class ListNode {
+
+        String url;
+        ListNode next;
+        ListNode prev;
+
+        public ListNode(String url) {
+            this.url = url;
+        }
+    }
+}