# Interleave the first half of the queue with the second half

Introduction

Queues are fundamental data structures used extensively in computer science and

daily applications. To interleave the first and second halves of a queue, reorganize the

queue's items so that the first and second halves alternate.

Example

Assume we have a queue that initially contains the integers 1, 2, 3, 4, and 5.

Original queue: 1, 2, 3, 4, 5.

To interleave the first and second halves of the queue, reorder the components so

that they alternate between the two.

1. Calculate half size:

• The line has a size of 5. Because 5 is an odd number, we may define the first half asfloor(size/2), which equals 2.

2. Dequeue and store the first half:

• Dequeue the first two entries (1 and 2) from the original queue and keep them in atemporary data structure.
• Temporary Queue: 1, 2.

3. Interleaving Elements:

• Enqueue the components from the temporary queue into the regular queue in alternating order with the remaining elements.
• Original queue after interleaving: 1, 3, 2, 4, 5.

Now, the queue's items are interleaved as required. The first half (1-2) alternates with the second half (3-4-5).

So the interleaved queue is 1, 3, 2, 4, 5.

It shows how the initial half of the queue has been interspersed with the second half.

Approach 1: Using Additional Data Structures

One method is to temporarily store the queue's elements using extra data structures such as arraysor linked lists. This is how it works.

1. Dequeue Half of the Elements: First, we remove half of the elements from the original queue. These dequeued elements are kept in a temporary data structure, such as an array or linked list. This step separates the line into two halves.
2. Interleave Remaining Elements: After dequeuing the first half of the elements, the remainder of the original queue is interleaved with the elements from the temporary data structure. This interleaving method entails first dequeuing an element from the main queue, then requeuing it, and finally dequeuing an element from the temporary data structure and requeuing it in the original queue. This process continues until he remaining elements from the original queue and the temporary data structure are enqueued.
3. Enqueue Elements from Temporary Structure: After all of the remaining elements from the original queue and the temporary data structure have been interleaved, the elements stored in the temporary data structure are requeued in the original queue. Similarly to the interleaving procedure in step 2, elements are dequeued from the temporary data structure and requeued in the original queue, where they interleave with the existing elements.
4. Time Complexity Analysis:
• This technique has a time complexity of O(n), where n is the number of entries in thequeue. It is because we only loop over the queue once to dequeue and enqueueelements.
• However, the space complexity is also O(n) because we store half of the items in atemporary data structure.

Implementation

Output:

Explanation

1. interleaveQueue Function:

• It takes a Queue<Integer> named queue as input.
• It calculates the size of the queue and divides it by 2 to find the index where the second half begins.
• It creates a temporary queue named tempQueue to store elements temporarily.
• It dequeues the first half of elements from the original queue and enqueues them into the temporary queue.
• It interleaves the elements from the temporary queue with the remaining elements from the original queue and enqueues them back into the original queue.

2. main Function:

• It initializes a LinkedList named queue and adds some integer elements to it.
• It calls the interleaveQueue function to interleave the elements of the queue.
• It prints the interleaved queue by dequeuing and also displaying each element.

Approach 2: Using Queue Operations

Another option is to use the operations offered by the queue data structure itself.

Here's how to do it:

1. Dequeue half of the elements and interleave them.

• We begin by dequeuing half of the elements in the original queue. It may beaccomplished by iterating over the queue and dequeuing elements until we have dequeued half of them.
• While dequeuing these items, we instantly requeue them in the proper interleavedsequence.
• For example, if we dequeued elements 1, 2, and 3 from the original queue, werequeue them as 1, 3, and 2, resulting in interleaving.

2. Dequeue and interleave the remaining elements.

• After interleaving the first half of the components, we continue dequeuing theremaining elements from the initial queue.
• Similar to the last phase, when we dequeue each element, we requeue it to theoriginal queue, interleaving it with the items that are already there.
• This interleaving guarantees that the second half of the parts are properly interleavedwith the first.

3. Time Complexity Analysis:

• This solution removes the need for new data structures but entails several dequeueand enqueue operations, resulting in a temporal complexity of O(n^2) in the worst-case situation.
• In the worst-case scenario, each dequeue and enqueue action takes O(n) time. Withpossibly O(n) such operations, the entire time complexity is O(n^2).

Implementation

Output:

Explanation

1. interleaveQueue Function:

• The interleaveQueue function takes a Queue named queue as input.
• It calculates the size of the queue and divides it by 2 to find the index where the second half begins.
• It then iterates over the first half of the queue and interleaves each element with the subsequent element from the second half. It is achieved by dequeuing an element from the first half and immediately enqueuing it back to the queue ,followed by dequeuing an element from the second half and enqueuing it back to the queue. This interleaving is done until the end of the first half.
• After interleaving the first half with the second half, the queue is now interleaved as desired.

2. main Function:

• In the main function, a LinkedList named queue is initialized and populated with integer elements.
• The interleaveQueue function is then called to interleave the elements of the queue.
• Finally, the interleaved queue is printed by dequeuing and displaying each element.

Approach 3: Recursive Approach.

A recursive strategy can be used to interleave the queue's items. Here is a high-level summary.

1. Recursive Dequeue and Interleaving:

• If the queue has two or more items, the recursive technique is used. First, the function dequeues half of the entries in the queue. It may beaccomplished by repeatedly invoking the function until the base case is encountered.
• Once the base case is reached, the function begins to return, interweaving andenqueuing the components back into the queue.
• This technique effectively interweaves the first half of the pieces recursively.

2. Dequeue and interleave the remaining elements.

• After repeatedly interleaving the first half of the data, the function removes the remainder from the queue.
• It then interleaves these remaining elements with the elements that are alreadypresent in the queue, ensuring that the second half is properly interleaved with thefirst half.
• Finally, it enqueues these interleaved remaining elements back into the queue.
• Finally, it enqueues these interleaved remaining elements back into the queue.

3. Time Complexity:

• This technique has a temporal complexity of O(n log n) owing to the repetitive calls.
• Each recursive call halves the issue size in half, resulting in a logarithmic number oflevels in the recursion tree.
• At each level, there is a linear time complexity operation (dequeuing and enqueuingitems), with a total time complexity of O(n log n).

Implementation

Output:

Explanation

• The interleaveQueue function starts by checking if the queue has fewer than two elements. If the queue has one or zero elements, it means no interleaving is necessary, so the function returns without making any changes to the queue.
• If the queue has two or more elements, the function proceeds to dequeue and enqueue the first half of the elements recursively.
• To do this, it dequeues elements from the queue and enqueues them into a temporary queue until half of the elements are dequeued.
• It then recursively calls itself on the remaining elements in the original queue, which repeats the process until the base case is reached.
• After the recursive calls return, the function interleaves the elements from the temporary queue with the elements remaining in the original queue.
• It does this by alternately dequeuing elements from the temporary queue and the original queue and enqueuing them back into the original queue.
• This interleaving process ensures that the first half of the elements is properly interleaved with the second half.
• Once the interleaving process is complete, the main function prints the elements of the interleaved queue by dequeuing and displaying each element.