Remove the last element in a queue - algorithm

Question

I need to remove the last element of a queue.
The only operations I may use are
Peek() - get the first element without removing it
Enqueue(element) - Insert an element to the back of the queue
Dequeue() - Remove the first element
IsEmpty() - true or false whether the queue is empty.
And I can't use arrays or queues, and the number of elements is not available.
I thought of some solutions, but each time I get stuck, since I don't know how to tell if the current element is the last element.

Answer 1

Justin Beal's solution is the more straight forward. But I think it can be done in place, without creating another queue.
object RemoveLast(Queue q) {
object first = q.Peek();
object current = null;
while (true) {
current = q.Dequeue();
if (q.Peek() == first) {
break;
}
q.Enqueue(current);
}
return current;
}

Answer 2

each time I get stuck since I don't know how to tell if the current element is the last element.
Good. Since you don't know of the "current" element is last, you do know of the "previous" element was last.
So, by saving the previous element, you should be able to handle this.

Answer 3

Something like this will work:
public Queue removeLast(Queue queue1){
Queue queue2 = new Queue();
while(!queue1.isEmpty()){
Object o = queue1.dequeue();
if(!queue1.isEmpty()){
queue2.enqueue(o);
}
}
return queue2;
}

Answer 4

I had the same requirement so have created an extension method that others may use if their google reached here.
using System.Collections.Generic;
using System.Linq;
/// <summary>
/// Extensions for <see cref="System.Collections.Generic.Queue{T}"/>.
/// </summary>
public static class QueueExtensionsStatic
{
/// <summary>
/// Removes the last item from the <see cref="System.Collections.Generic.Queue{T}"/>.
/// </summary>
/// <typeparam name="T">Type of object in <see cref="System.Collections.Generic.Queue{T}"/>.</typeparam>
/// <param name="q">Instance of <see cref="System.Collections.Generic.Queue{T}"/> to remove item from.</param>
/// <returns>Item removed from the <see cref="System.Collections.Generic.Queue{T}"/>.</returns>
public static T Pop<T>(this Queue<T> q)
{
for (var i = 1; i < q.Count; i++)
q.Enqueue(q.Dequeue());
return q.Dequeue();
}
/// <summary>
/// Removes the last <see cref="quantity"/> item(s) from the <see cref="System.Collections.Generic.Queue{T}"/>.
/// </summary>
/// <typeparam name="T">Type of object in <see cref="System.Collections.Generic.Queue{T}"/>.</typeparam>
/// <param name="q">Instance of <see cref="System.Collections.Generic.Queue{T}"/> to remove item from.</param>
/// <param name="quantity">Number of items to pop off the end of the <see cref="System.Collections.Generic.Queue{T}"/>.</param>
/// <returns>Item removed from the <see cref="System.Collections.Generic.Queue{T}"/>.</returns>
public static IEnumerable<T> Pop<T>(this Queue<T> q, int quantity)
{
for (var i = quantity; i < q.Count; i++)
q.Enqueue(q.Dequeue());
var poppedItems = new List<T>(quantity);
for (int i = 0; i < quantity; i++)
poppedItems.Add(q.Dequeue());
return poppedItems;
}
/// <summary>
/// Adds an item(<see cref="T"/>) to the start of the <see cref="System.Collections.Generic.Queue{T}"/>.
/// </summary>
/// <typeparam name="T">Type of object in <see cref="System.Collections.Generic.Queue{T}"/>.</typeparam>
/// <param name="q">Instance of <see cref="System.Collections.Generic.Queue{T}"/> to remove item from.</param>
public static void Push<T>(this Queue<T> q, T item)
{
q.Enqueue(item);
for (var i = 1; i < q.Count; i++)
q.Enqueue(q.Dequeue());
}
/// <summary>
/// Adds items(<see cref="T"/>) to the start of the <see cref="System.Collections.Generic.Queue{T}"/>.
/// </summary>
/// <typeparam name="T">Type of object in <see cref="System.Collections.Generic.Queue{T}"/>.</typeparam>
/// <param name="q">Instance of <see cref="System.Collections.Generic.Queue{T}"/> to remove item from.</param>
/// <param name="items">List of items(<see cref="T"/>) to add to the <see cref="System.Collections.Generic.Queue{T}"/>.</param>
public static void Push<T>(this Queue<T> q, IEnumerable<T> items)
{
if (items == null || !items.Any()) return;
foreach (var item in items)
q.Enqueue(item);
for (var i = items.Count(); i < q.Count; i++)
q.Enqueue(q.Dequeue());
}
}

Answer 5

I faced the same problem a couple days ago while implementing a similar requirement for my assignment. The simplest method to this problem i believe is looping through the queue and pushing the 'front' element to the back of the queue and 'pop' it , but when you reach the last element in the queue just 'pop' it and don't 'push' it.

Answer 6

I went through the solution provided by Trinidad, which is the most voted answer so far. I guess this solution works perfectly only if the first element/object in the Queue is unique. If any duplicate of the first element/object is encountered, it fails to give the correct element as the last element.