Queues in Java (#218)

* Clone linked list with random pointer

* addition in  reviewers list

* Queues in Java

queues/README.md

@@ -5,3 +5,8 @@
 1. [Queue using linked list](c-or-cpp/queue-linked-list.cpp)
 2. [Circular Queue using linked list](c-or-cpp/circular-queue-linked-list.cpp)
 3. [Double Ended Queue (using arrays)](c-or-cpp/double-ended-queue-using-array.c)
+
+### Java
+
+1. [Queue using linked list](java/queue-linked-list.java)
+2. [Circular Queue using linked list](java/circular-queue-linked-list.java)

queues/java/circular-queue-linked-list.java

@@ -0,0 +1,96 @@
+// Java program for insertion and
+// deletion in Circular Queue
+import java.util.*;
+  
+class Solution {
+  
+    // Structure of a Node
+    static class Node {
+        int data;
+        Node link;
+    }
+  
+    static class Queue {
+        Node front, rear;
+    }
+  
+    // Function to create Circular queue
+    static void enQueue(Queue q, int value)
+    {
+        Node temp = new Node();
+        temp.data = value;
+        if (q.front == null)
+            q.front = temp;
+        else
+            q.rear.link = temp;
+  
+        q.rear = temp;
+        q.rear.link = q.front;
+    }
+  
+    // Function to delete element from Circular Queue
+    static int deQueue(Queue q)
+    {
+        if (q.front == null) {
+            System.out.printf("Queue is empty");
+            return Integer.MIN_VALUE;
+        }
+  
+        // If this is the last node to be deleted
+        int value; // Value to be dequeued
+        if (q.front == q.rear) {
+            value = q.front.data;
+            q.front = null;
+            q.rear = null;
+        }
+        else // There are more than one nodes
+        {
+            Node temp = q.front;
+            value = temp.data;
+            q.front = q.front.link;
+            q.rear.link = q.front;
+        }
+  
+        return value;
+    }
+  
+    // Function displaying the elements of Circular Queue
+    static void displayQueue(Queue q)
+    {
+        Node temp = q.front;
+        System.out.printf("\nElements in Circular Queue are: ");
+        while (temp.link != q.front) {
+            System.out.printf("%d ", temp.data);
+            temp = temp.link;
+        }
+        System.out.printf("%d", temp.data);
+    }
+  
+    /* Driver of the program */
+    public static void main(String args[])
+    {
+        // Create a queue and initialize front and rear
+        Queue q = new Queue();
+        q.front = q.rear = null;
+  
+        // Inserting elements in Circular Queue
+        enQueue(q, 14);
+        enQueue(q, 22);
+        enQueue(q, 6);
+  
+        // Display elements present in Circular Queue
+        displayQueue(q);
+  
+        // Deleting elements from Circular Queue
+        System.out.printf("\nDeleted value = %d", deQueue(q));
+        System.out.printf("\nDeleted value = %d", deQueue(q));
+  
+        // Remaining elements in Circular Queue
+        displayQueue(q);
+  
+        enQueue(q, 9);
+        enQueue(q, 20);
+        displayQueue(q);
+    }
+}
+  

queues/java/queue-linked-list.java

@@ -0,0 +1,78 @@
+// Java program for linked-list implementation of queue
+  
+// A linked list (LL) node to store a queue entry
+class QNode {
+    int key;
+    QNode next;
+  
+    // constructor to create a new linked list node
+    public QNode(int key)
+    {
+        this.key = key;
+        this.next = null;
+    }
+}
+  
+// A class to represent a queue
+// The queue, front stores the front node of LL and rear stores the
+// last node of LL
+class Queue {
+    QNode front, rear;
+  
+    public Queue()
+    {
+        this.front = this.rear = null;
+    }
+  
+    // Method to add an key to the queue.
+    void enqueue(int key)
+    {
+  
+        // Create a new LL node
+        QNode temp = new QNode(key);
+  
+        // If queue is empty, then new node is front and rear both
+        if (this.rear == null) {
+            this.front = this.rear = temp;
+            return;
+        }
+  
+        // Add the new node at the end of queue and change rear
+        this.rear.next = temp;
+        this.rear = temp;
+    }
+  
+    // Method to remove an key from queue.
+    void dequeue()
+    {
+        // If queue is empty, return NULL.
+        if (this.front == null)
+            return;
+  
+        // Store previous front and move front one node ahead
+        QNode temp = this.front;
+        this.front = this.front.next;
+  
+        // If front becomes NULL, then change rear also as NULL
+        if (this.front == null)
+            this.rear = null;
+    }
+}
+  
+// Driver class
+public class Test {
+    public static void main(String[] args)
+    {
+        Queue q = new Queue();
+        q.enqueue(10);
+        q.enqueue(20);
+        q.dequeue();
+        q.dequeue();
+        q.enqueue(30);
+        q.enqueue(40);
+        q.enqueue(50);
+        q.dequeue();
+        System.out.println("Queue Front : " + q.front.key);
+        System.out.println("Queue Rear : " + q.rear.key);
+    }
+}