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Data Structures for Coding Interviews in C#

Solution Review: Reversing First "k" Elements of Queue

This review provides a detailed analysis to help you solve the "Reversing First "k" Elements of Queue" challenge.

We'll cover the following...

Solution: Using a queue #

C#
using System;
using System.Collections;
namespace chapter_4
{
    class Challenge_3
    {
        static Queue reverseK(Queue queue, int k)
        {
            //1.Push first k elements in queue in a stack.
            //2.Pop Stack elements and enqueue them at the end of queue
            //3.Dequeue queue elements till "k" and append them at the end of queue  
            if (queue.Count != 0)
            {
                Stack stack =  new Stack();
                int count = 0;
                while (count < k)
                {
                    stack.Push(queue.Dequeue());
                    count++;
                }
                while (stack.Count != 0)
                {
                    queue.Enqueue(stack.Pop());
                }
                int size = queue.Count;
                for (int i = 0; i < size - k; i++)
                {
                    queue.Enqueue(queue.Dequeue());
                }
            }
            return queue;
        }
        static  void Showqueue(Queue s)
        {
            Console.WriteLine("The queue is: ");
            while (s.Count != 0)
            {
                Console.Write('\t' + s.Peek().ToString());
                s.Dequeue();
            }
            Console.WriteLine('\n');
        }
        static void Main(string[] args)
        {
            Queue mQ = new Queue();
            mQ.Enqueue(1);
            mQ.Enqueue(2);
            mQ.Enqueue(3);
            mQ.Enqueue(4);
            mQ.Enqueue(5);
            mQ = reverseK(mQ, 5);
            Showqueue(mQ);
            return ;
        }
    }
}

Take a look at the algorithm step-by-step:

  1. Check if the queue is empty (line 14). If it is not, start by creating a stack. The available stack functions are:

    • Constructor: myStack()
    • Push elements: push(int) to add elements to the stack
    • Pop elements: pop() to remove or pop the top element from the stack.
    • Check if empty: isEmpty() returns true if the stack is empty and false otherwise.
    • Return top: getTop() function returns the top element (that has been added at the end) without removing it from the stack.

  2. The function reverseK(queue, k) takes ...