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Append the elements of queue in mirror-inverse order
  • Difficulty Level : Easy
  • Last Updated : 11 Dec, 2020

Given a queue Q containing N strings, the task is to restructure the queue to double its size such that the second half represents the mirror image of the first half.
Examples:  

Input: Q = {“Hello”, “World”} 
Output: {“Hello”, “World”, “World”, “Hello”} 
Explanation: 
The second half of the output queue is the mirror image of the first half. That is: 
“Hello”, “World” | “World”, “Hello”
Input: Q = {“Hi”, “Geeks”} 
Output: {“Hi”, “Geeks”, “Geeks”, “Hi”} 
 

Approach: On observing carefully, we can say that the second half of the output queue is the reverse of the first half. That is:  

  1. Store the size of the queue in a variable named size.
  2. Push the Queue elements into a stack without actually losing the elements. This can be achieved by using emplace().
  3. Repeat the process until size becomes 0.
  4. Push the elements of the stack back to the queue.

Below is the implementation of the above approach. 

C++

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// C++ program to arrange the
// elements of the queue
// to the end such that
// the halves are mirror
// order of each other
#include <bits/stdc++.h>
using namespace std;
 
// Function to display
// the elements of
// the queue
void showq(queue<string> q)
{
    // Iterating through the queue
    // and printing the elements
    while (!q.empty()) {
        cout << q.front() << " ";
        q.pop();
    }
}
 
// Function to produce mirror elements
void mirrorQueue(queue<string>& q)
{
    int size = q.size();
 
    // Defining a stack
    stack<string> st;
 
    // Pushing the elements
    // of a queue
    // in a stack without
    // losing them
    // from the queue
    while (size--) {
        string x = q.front();
        // Push the element
        // to the end of the
        // queue
        q.emplace(x);
        // Push the element
        // into the stack
        st.push(x);
        // Remove the element
        q.pop();
    }
 
    // Appending the elements
    // from the stack
    // to the queue
    while (!st.empty()) {
        string el = st.top();
        q.push(el);
        st.pop();
    }
}
 
// Driver Code
int main()
{
    queue<string> q;
    q.push("Hello");
    q.push("World");
    mirrorQueue(q);
    showq(q);
    return 0;
}

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Java

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// Java program to arrange the
// elements of the queue
// to the end such that
// the halves are mirror
// order of each other
import java.util.*;
class GFG{
 
// Function to display
// the elements of
// the queue
static void showq(Queue<String> q)
{
    // Iterating through the queue
    // and printing the elements
    while (!q.isEmpty())
    {
        System.out.print(q.peek() + " ");
        q.remove();
    }
}
 
// Function to produce mirror elements
static void mirrorQueue(Queue<String> q)
{
    int size = q.size();
 
    // Defining a stack
    Stack<String> st = new Stack<>();
 
    // Pushing the elements
    // of a queue
    // in a stack without
    // losing them
    // from the queue
    while (size-->0)
    {
        String x = q.peek();
       
        // Push the element
        // to the end of the
        // queue
        q.add(x);
       
        // Push the element
        // into the stack
        st.add(x);
       
        // Remove the element
        q.remove();
    }
 
    // Appending the elements
    // from the stack
    // to the queue
    while (!st.isEmpty())
    {
        String el = st.peek();
        q.add(el);
        st.pop();
    }
}
 
// Driver Code
public static void main(String[] args)
{
    Queue<String> q =
          new inkedList<String>();
    q.add("Hello");
    q.add("World");
    mirrorQueue(q);
    showq(q);
}
}
 
// This code is contributed by gauravrajput1

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Python3

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# Python3 program to arrange the
# elements of the queue
# to the end such that
# the halves are mirror
# order of each other
 
# Function to display
# the elements of
# the queue
def showq(q) :
 
    # Iterating through the queue
    # and printing the elements
    while (len(q) != 0) :
     
        print(q[0] , end = " ")
        q.pop(0)
         
# Function to produce mirror elements
def mirrorQueue(q) :
 
    size = len(q)
  
    # Defining a stack
    st = []
  
    # Pushing the elements
    # of a queue
    # in a stack without
    # losing them
    # from the queue
    while (size > 0) :
     
        x = q[0]
        
        # Push the element
        # to the end of the
        # queue
        q.append(x)
        
        # Push the element
        # into the stack
        st.append(x)
        
        # Remove the element
        q.pop(0)
         
        size -= 1
  
    # Appending the elements
    # from the stack
    # to the queue
    while (len(st) != 0) :
     
        el = st[len(st) - 1]
        q.append(el)
        st.pop()
         
q = []
q.append("Hello")
q.append("World")
mirrorQueue(q)
showq(q)
 
# This code is contributed by divyeshrabadiya07

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C#

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// C# program to arrange the
// elements of the queue
// to the end such that
// the halves are mirror
// order of each other
using System;
using System.Collections.Generic;
class GFG{
 
// Function to display
// the elements of
// the queue
static void showq(Queue<String> q)
{
  // Iterating through the queue
  // and printing the elements
  while (q.Count != 0)
  {
    Console.Write(q.Peek() + " ");
    q.Dequeue();
  }
}
 
// Function to produce mirror elements
static void mirrorQueue(Queue<String> q)
{
  int size = q.Count;
 
  // Defining a stack
  Stack<String> st = new Stack<String>();
 
  // Pushing the elements
  // of a queue
  // in a stack without
  // losing them
  // from the queue
  while (size --> 0)
  {
    String x = q.Peek();
 
    // Push the element
    // to the end of the
    // queue
    q.Enqueue(x);
 
    // Push the element
    // into the stack
    st.Push(x);
 
    // Remove the element
    q.Dequeue();
  }
 
  // Appending the elements
  // from the stack
  // to the queue
  while (st.Count != 0)
  {
    String el = st.Peek();
    q.Enqueue(el);
    st.Pop();
  }
}
 
// Driver Code
public static void Main(String[] args)
{
  Queue<String> q = new Queue<String>();
  q.Enqueue("Hello");
  q.Enqueue("World");
  mirrorQueue(q);
  showq(q);
}
}
 
// This code is contributed by Rajput-Ji

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Output: 

Hello World World Hello

 

Time Complexity: O(N), where N is the size of the queue. 

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