Suppose you want to create a copy of a queue, Q, without destroying the original queue.
If you simply do Queue C = Q , it will not create a true copy, it just creates a second reference to Q – if Q is changed, C will also change in this case.
 
Here’s one way to do this:
1. Make two empty queues, C and T.
2. while Q is not empty do:
Dequeue Q, called this element e. Make a copy of e, call it e2.
Enqueue e in C, enqueue e2 in T.
3. While T is not empty:
Dequeue T, and enqueue the dequeued element in C
4. C is an identical copy of Q. This is called a deep copy of Q.
Basically we are dequeuing Q, copying the elements to C and keeping a temporary queue T. Then we dequeue the temporary queue T and enqueue into Q.
If we don’t do this, your oroiginal Q will be empty afterwards. Draw some pictures and see for yourself.
Write a method,
public static LinkedList<Integer> copyOfQ(LinkedList<Integer> Q)
to return a copy of an input Queue Q of Integer objects. We are crating a Queue here using JDK’s java.util.LinkedList class.
import java.util.*;
public class CopyQ{
public static LinkedList<Integer> copyOfQ(LinkedList<Integer> Q){
//Your Code below
}
public static void main(String[] args){
LinkedList<Integer> Q= new LinkedList<>();
for (int i=0; i<10; i++){
Q.add(new Integer(i+1));
}
LinkedList<Integer> C= copyOfQ(Q);
System.out.println(“Checking Original Queue:”);
for(Integer e:Q){
System.out.print(e+” “);
}
System.out.println();
System.out.println(“Checking Copy:”);
for(Integer e:C){
System.out.print(e+” “);
}
System.out.println();
}
}