added java file for queue data structure

Signed-off-by: Shreyansh Khaitan <[email protected]>

reverse a queue

@@ -0,0 +1,239 @@
+//using stack
+import java.util.*;
+class Queue
+{
+  int front, rear, size;
+  int capacity;
+  int[] array;
+
+  public Queue (int capacity)
+  {
+    this.capacity = capacity;
+    front = this.size = 0;
+    rear = capacity - 1;
+    array = new int[this.capacity];
+  }
+
+  // Queue is full when size becomes
+  // equal to the capacity
+  boolean isFull (Queue queue)
+  {
+    return (queue.size == queue.capacity);
+  }
+
+  // Queue is empty when size is 0
+  boolean isEmpty (Queue queue)
+  {
+    return (queue.size == 0);
+  }
+
+  // Method to add an item to the queue.
+  // It changes rear and size
+  void enqueue (int item)
+  {
+    if (isFull (this))
+      return;
+    this.rear = (this.rear + 1) % this.capacity;
+    this.array[this.rear] = item;
+    this.size = this.size + 1;
+
+  }
+  int dequeue ()
+  {
+    if (isEmpty (this))
+      return Integer.MIN_VALUE;
+
+    int item = this.array[this.front];
+    this.front = (this.front + 1) % this.capacity;
+    this.size = this.size - 1;
+    return item;
+  }
+
+
+
+  public Queue queuereverse (Queue q)
+  {
+    Stack < Integer > stk = new Stack <> ();
+    while (!q.isEmpty (q))
+      {
+	stk.add (q.array[front]);
+	q.dequeue ();
+      }
+    while (!stk.isEmpty ())
+      {
+	q.enqueue (stk.peek ());
+	stk.pop ();
+      }
+    return q;
+  }
+
+  public void disp () /* function to display the elements of the queue */
+  {
+
+    if (front == -1)
+      {
+	System.out.print ("\nQueue is Empty");
+      }
+    else
+      {
+	for (int i = front; i <= rear; i++)
+	  {
+	    System.out.print (array[i] + "  ");
+	  }
+      }
+    System.out.println ();
+  }
+
+}
+
+// Driver class
+public class Main
+{
+  public static void main (String[]args)
+  {
+    Queue queue = new Queue (1000);
+
+      queue.enqueue (10);
+      queue.enqueue (20);
+      queue.enqueue (30);
+      queue.enqueue (40);
+
+      System.out.println ("Original queue : ");
+      queue.disp ();
+      System.out.println ();
+
+      queue = queue.queuereverse (queue);
+
+      System.out.println ("Queue after reversing : ");
+      queue.disp ();
+
+      System.out.println ();
+
+
+  }
+}
+
+//using recursion
+import java.util.*;
+class Queue
+{
+  int front, rear, size;
+  int capacity;
+  int array[];
+
+  public Queue (int capacity)
+  {
+    this.capacity = capacity;
+    front = this.size = 0;
+    rear = capacity - 1;
+    array = new int[this.capacity];
+  }
+
+  // Queue is full when size becomes
+  // equal to the capacity
+  boolean isFull (Queue queue)
+  {
+    return (queue.size == queue.capacity);
+  }
+
+  // Queue is empty when size is 0
+  boolean isEmpty (Queue queue)
+  {
+    return (queue.size == 0);
+  }
+
+  // Method to add an item to the queue.
+  // It changes rear and size
+  void enqueue (int item)
+  {
+    if (isFull (this))
+      return;
+    this.rear = (this.rear + 1) % this.capacity;
+    this.array[this.rear] = item;
+    this.size = this.size + 1;
+
+  }
+  int dequeue ()
+  {
+    if (isEmpty (this))
+      return Integer.MIN_VALUE;
+
+    int item = this.array[this.front];
+    this.front = (this.front + 1) % this.capacity;
+    this.size = this.size - 1;
+    return item;
+  }
+
+  public Queue reverseQueue (Queue q)
+  {
+    // Base case
+    if (q.isEmpty (q))
+      return q;
+
+    // Dequeue current item
+    int data = q.array[front];
+    q.dequeue ();
+
+    // Reverse remaining queue
+    q = reverseQueue (q);
+
+    // Enqueue current item
+    q.enqueue (data);
+
+    return q;
+  }
+
+  public void disp () /* function to display the elements of the queue */
+  {
+
+    if (front == -1)
+      {
+	System.out.print ("\nQueue is Empty");
+      }
+    else
+      {
+	for (int i = front; i <= rear; i++)
+	  {
+	    System.out.print (array[i] + "  ");
+	  }
+      }
+    System.out.println ();
+  }
+
+}
+
+// Driver class
+public class Main
+{
+  public static void main (String[]args)
+  {
+    Queue queue = new Queue (1000);
+
+      queue.enqueue (10);
+      queue.enqueue (20);
+      queue.enqueue (30);
+      queue.enqueue (40);
+
+      System.out.println ("Original queue : ");
+      queue.disp ();
+      System.out.println ();
+
+      queue.reverseQueue (queue);
+
+      System.out.println ("Queue after reversing : ");
+      queue.disp ();
+
+      System.out.println ();
+
+
+  }
+}
+
+/*
+ * Output: 
+Original queue : 
+10  20  30  40  
+
+Queue after reversing : 
+40  30  20  10
+ */