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Interleave the first half of the queue with second half

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  • Difficulty Level : Medium
  • Last Updated : 28 Jun, 2022

Given a queue of integers of even length, rearrange the elements by interleaving the first half of the queue with the second half of the queue.  

Examples:

Input :  1 2 3 4
Output : 1 3 2 4

Input : 11 12 13 14 15 16 17 18 19 20
Output : 11 16 12 17 13 18 14 19 15 20

Using Stack

Following are the steps to solve the problem: 

  1. Push the first half elements of the queue to stack. 
  2. Enqueue back the stack elements. 
  3. Dequeue the first half elements of the queue and enqueue them back. 
  4. Again push the first half elements into the stack. 
  5. Interleave the elements of queue and stack. 

C++




// C++ program to interleave the first half of the queue
// with the second half
#include <bits/stdc++.h>
using namespace std;
 
// Function to interleave the queue
void interLeaveQueue(queue<int>& q)
{
    // To check the even number of elements
    if (q.size() % 2 != 0)
        cout << "Input even number of integers." << endl;
 
    // Initialize an empty stack of int type
    stack<int> s;
    int halfSize = q.size() / 2;
 
    // Push first half elements into the stack
    // queue:16 17 18 19 20, stack: 15(T) 14 13 12 11
    for (int i = 0; i < halfSize; i++) {
        s.push(q.front());
        q.pop();
    }
 
    // enqueue back the stack elements
    // queue: 16 17 18 19 20 15 14 13 12 11
    while (!s.empty()) {
        q.push(s.top());
        s.pop();
    }
 
    // dequeue the first half elements of queue
    // and enqueue them back
    // queue: 15 14 13 12 11 16 17 18 19 20
    for (int i = 0; i < halfSize; i++) {
        q.push(q.front());
        q.pop();
    }
 
    // Again push the first half elements into the stack
    // queue: 16 17 18 19 20, stack: 11(T) 12 13 14 15
    for (int i = 0; i < halfSize; i++) {
        s.push(q.front());
        q.pop();
    }
 
    // interleave the elements of queue and stack
    // queue: 11 16 12 17 13 18 14 19 15 20
    while (!s.empty()) {
        q.push(s.top());
        s.pop();
        q.push(q.front());
        q.pop();
    }
}
 
// Driver program to test above function
int main()
{
    queue<int> q;
    q.push(11);
    q.push(12);
    q.push(13);
    q.push(14);
    q.push(15);
    q.push(16);
    q.push(17);
    q.push(18);
    q.push(19);
    q.push(20);
    interLeaveQueue(q);
    int length = q.size();
    for (int i = 0; i < length; i++) {
        cout << q.front() << " ";
        q.pop();
    }
    return 0;
}


Java




// Java program to interleave
// the first half of the queue
// with the second half
import java.util.*;
 
class GFG {
 
    // Function to interleave the queue
    static void interLeaveQueue(Queue<Integer> q)
    {
        // To check the even number of elements
        if (q.size() % 2 != 0)
            System.out.println(
                "Input even number of integers.");
 
        // Initialize an empty stack of int type
        Stack<Integer> s = new Stack<>();
        int halfSize = q.size() / 2;
 
        // Push first half elements into the stack
        // queue:16 17 18 19 20, stack: 15(T) 14 13 12 11
        for (int i = 0; i < halfSize; i++) {
            s.push(q.peek());
            q.poll();
        }
 
        // enqueue back the stack elements
        // queue: 16 17 18 19 20 15 14 13 12 11
        while (!s.empty()) {
            q.add(s.peek());
            s.pop();
        }
 
        // dequeue the first half elements of queue
        // and enqueue them back
        // queue: 15 14 13 12 11 16 17 18 19 20
        for (int i = 0; i < halfSize; i++) {
            q.add(q.peek());
            q.poll();
        }
 
        // Again push the first half elements into the stack
        // queue: 16 17 18 19 20, stack: 11(T) 12 13 14 15
        for (int i = 0; i < halfSize; i++) {
            s.push(q.peek());
            q.poll();
        }
 
        // interleave the elements of queue and stack
        // queue: 11 16 12 17 13 18 14 19 15 20
        while (!s.empty()) {
            q.add(s.peek());
            s.pop();
            q.add(q.peek());
            q.poll();
        }
    }
 
    // Driver code
    public static void main(String[] args)
    {
        Queue<Integer> q = new java.util.LinkedList<>();
        q.add(11);
        q.add(12);
        q.add(13);
        q.add(14);
        q.add(15);
        q.add(16);
        q.add(17);
        q.add(18);
        q.add(19);
        q.add(20);
        interLeaveQueue(q);
        int length = q.size();
        for (int i = 0; i < length; i++) {
            System.out.print(q.peek() + " ");
            q.poll();
        }
    }
}
 
// This code contributed by Rajput-Ji


Python3




# Python3 program to interleave the first
# half of the queue with the second half
from queue import Queue
 
# Function to interleave the queue
 
 
def interLeaveQueue(q):
 
    # To check the even number of elements
    if (q.qsize() % 2 != 0):
        print("Input even number of integers.")
 
    # Initialize an empty stack of int type
    s = []
    halfSize = int(q.qsize() / 2)
 
    # put first half elements into
    # the stack queue:16 17 18 19 20,
    # stack: 15(T) 14 13 12 11
    for i in range(halfSize):
        s.append(q.queue[0])
        q.get()
 
    # enqueue back the stack elements
    # queue: 16 17 18 19 20 15 14 13 12 11
    while len(s) != 0:
        q.put(s[-1])
        s.pop()
 
    # dequeue the first half elements of
    # queue and enqueue them back
    # queue: 15 14 13 12 11 16 17 18 19 20
    for i in range(halfSize):
        q.put(q.queue[0])
        q.get()
 
    # Again put the first half elements into
    # the stack queue: 16 17 18 19 20,
    # stack: 11(T) 12 13 14 15
    for i in range(halfSize):
        s.append(q.queue[0])
        q.get()
 
    # interleave the elements of queue and stack
    # queue: 11 16 12 17 13 18 14 19 15 20
    while len(s) != 0:
        q.put(s[-1])
        s.pop()
        q.put(q.queue[0])
        q.get()
 
 
# Driver Code
if __name__ == '__main__':
    q = Queue()
    q.put(11)
    q.put(12)
    q.put(13)
    q.put(14)
    q.put(15)
    q.put(16)
    q.put(17)
    q.put(18)
    q.put(19)
    q.put(20)
    interLeaveQueue(q)
    length = q.qsize()
    for i in range(length):
        print(q.queue[0], end=" ")
        q.get()
 
# This code is contributed by PranchalK


C#




// C# program to interleave
// the first half of the queue
// with the second half
using System;
using System.Collections.Generic;
 
class GFG {
 
    // Function to interleave the queue
    static void interLeaveQueue(Queue<int> q)
    {
        // To check the even number of elements
        if (q.Count % 2 != 0)
            Console.WriteLine(
                "Input even number of integers.");
 
        // Initialize an empty stack of int type
        Stack<int> s = new Stack<int>();
        int halfSize = q.Count / 2;
 
        // Push first half elements into the stack
        // queue:16 17 18 19 20, stack: 15(T) 14 13 12 11
        for (int i = 0; i < halfSize; i++) {
            s.Push(q.Peek());
            q.Dequeue();
        }
 
        // enqueue back the stack elements
        // queue: 16 17 18 19 20 15 14 13 12 11
        while (s.Count != 0) {
            q.Enqueue(s.Peek());
            s.Pop();
        }
 
        // dequeue the first half elements of queue
        // and enqueue them back
        // queue: 15 14 13 12 11 16 17 18 19 20
        for (int i = 0; i < halfSize; i++) {
            q.Enqueue(q.Peek());
            q.Dequeue();
        }
 
        // Again push the first half elements into the stack
        // queue: 16 17 18 19 20, stack: 11(T) 12 13 14 15
        for (int i = 0; i < halfSize; i++) {
            s.Push(q.Peek());
            q.Dequeue();
        }
 
        // interleave the elements of queue and stack
        // queue: 11 16 12 17 13 18 14 19 15 20
        while (s.Count != 0) {
            q.Enqueue(s.Peek());
            s.Pop();
            q.Enqueue(q.Peek());
            q.Dequeue();
        }
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        Queue<int> q = new Queue<int>();
        q.Enqueue(11);
        q.Enqueue(12);
        q.Enqueue(13);
        q.Enqueue(14);
        q.Enqueue(15);
        q.Enqueue(16);
        q.Enqueue(17);
        q.Enqueue(18);
        q.Enqueue(19);
        q.Enqueue(20);
        interLeaveQueue(q);
        int length = q.Count;
        for (int i = 0; i < length; i++) {
            Console.Write(q.Peek() + " ");
            q.Dequeue();
        }
    }
}
 
// This code is contributed by Princi Singh


Javascript




<script>
 
// JavaScript program to interleave the first
// half of the queue with the second half
 
// Function to interleave the queue
function interLeaveQueue(q){
     
    // To check the even number of elements
    if (q.length % 2 != 0)
        document.write("Inpush even number of integers.")
 
    // Initialize an empty stack of int type
    let s = []
    let halfSize = Math.floor(q.length / 2)
 
    // push first half elements into
    // the stack queue:16 17 18 19 20,
    // stack: 15(T) 14 13 12 11
    for(let i = 0; i < halfSize; i++)
        s.push(q.shift())
 
    // enqueue back the stack elements
    // queue: 16 17 18 19 20 15 14 13 12 11
    while(s.length != 0){
        q.push(s[s.length-1])
        s.pop()
    }
 
    // dequeue the first half elements of
    // queue and enqueue them back
    // queue: 15 14 13 12 11 16 17 18 19 20
    for(let i = 0; i < halfSize; i++)
        q.push(q.shift())
 
    // Again push the first half elements into
    // the stack queue: 16 17 18 19 20,
    // stack: 11(T) 12 13 14 15
    for(let i = 0; i < halfSize; i++)
        s.push(q.shift())
 
    // interleave the elements of queue and stack
    // queue: 11 16 12 17 13 18 14 19 15 20
    while(s.length != 0){
        q.push(s[s.length-1])
        s.pop()
        q.push(q.shift())
    }
}
 
// Driver Code
 
let q =[]
q.push(11)
q.push(12)
q.push(13)
q.push(14)
q.push(15)
q.push(16)
q.push(17)
q.push(18)
q.push(19)
q.push(20)
interLeaveQueue(q)
let length = q.length
for(let i=0;i<length;i++)
    document.write(q.shift()," ")
 
// This code is contributed by shinjanpatra
 
</script>


Output

11 16 12 17 13 18 14 19 15 20 

Time complexity: O(n). Auxiliary Space: O(n).

Using Queue

We can also solve the given problem by using a queue instead of a stack. The idea is to move the first half to another queue, and then push values from the temporary queue and original queue into the original queue. The original queue will get converted to the interleaved queue after the operations.

Steps to solve : 

  1. Make a temporary queue and push the first half of the original queue into the temp queue.
  2. Till the temp queue is empty
    1. Pop the front of the temp queue and push it to the original queue
    2. Pop the front of the original queue and push it to the original queue
  3. The original queue is converted to the interleaved queue.

Move the first half to the temporary queue

Interleaving the two halves

C++




// C++ program to interleave the first half of the queue
// with the second half using queue
#include <bits/stdc++.h>
using namespace std;
 
// Function to interleave the queue
void interLeaveQueue(queue<int>& q)
{
    // To check the even number of elements
    if (q.size() % 2 != 0)
        cout << "Input even number of integers." << endl;
 
    // Initialize an empty queue of int type
    queue<int> temp;
    int halfSize = q.size() / 2;
 
    // Push first half elements into the stack
    // queue:16 17 18 19 20, queue: 11 12 13 14 15
    for (int i = 0; i < halfSize; i++) {
        temp.push(q.front());
        q.pop();
    }
 
    // enqueue back the queue elements alternatively
    // queue: 11 16 12 17 13 18 14 19 15 20
    while (!temp.empty()) {
        q.push(temp.front());
        q.push(q.front());
        q.pop();
        temp.pop();
    }
}
 
// Driver program to test above function
int main()
{
    queue<int> q;
    q.push(11);
    q.push(12);
    q.push(13);
    q.push(14);
    q.push(15);
    q.push(16);
    q.push(17);
    q.push(18);
    q.push(19);
    q.push(20);
    interLeaveQueue(q);
    int length = q.size();
    for (int i = 0; i < length; i++) {
        cout << q.front() << " ";
        q.pop();
    }
    return 0;
}


Output

11 16 12 17 13 18 14 19 15 20 

Time complexity: O(n)  Auxiliary Space: O(n).

This article is contributed by Prakriti Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or if you want to share more information about the topic discussed above.


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