04_Queue.cs

using System;
using System.Collections.Generic;

namespace DataStructures
{
    /// <summary>
    /// Demonstrates the Queue<T> collection in C# (FIFO — First In, First Out).
    /// Elements are enqueued at the back and dequeued from the front.
    /// 
    /// Common use cases: task scheduling, breadth-first search (BFS),
    /// print job queues, message processing pipelines.
    /// </summary>
    public class QueueExamples
    {
        // ─── Entry Point ───────────────────────────────────────────────
        public static void Main()
        {
            Console.WriteLine("═══════════════════════════════════════════════");
            Console.WriteLine("            C# Queue<T> Examples               ");
            Console.WriteLine("═══════════════════════════════════════════════\n");

            // ── 1. Basic Queue Operations ───────────────────────────────
            Console.WriteLine("── Basic Queue Operations ─────────────────────");
            var tasks = new Queue<string>();

            // Enqueue — add elements to the back of the queue
            tasks.Enqueue("Download files");
            tasks.Enqueue("Parse data");
            tasks.Enqueue("Generate report");
            tasks.Enqueue("Send email notification");

            Console.WriteLine($"  Queue contents: {string.Join(" → ", tasks)}");
            Console.WriteLine($"  Count: {tasks.Count}");
            Console.WriteLine();

            // Peek — view the front element without removing it
            Console.WriteLine("── Peek & Dequeue ─────────────────────────────");
            Console.WriteLine($"  Peek()  → {tasks.Peek()}  (queue unchanged)");
            Console.WriteLine($"  Count   → {tasks.Count}");
            Console.WriteLine();

            // Dequeue — remove and return the front element (FIFO)
            while (tasks.Count > 0)
            {
                string task = tasks.Dequeue();
                Console.WriteLine($"  Dequeued: \"{task}\" | Remaining: {tasks.Count}");
            }
            Console.WriteLine();

            // ── 2. Interactive Vowel Processing Example ─────────────────
            Console.WriteLine("── Vowel Enqueue / Dequeue ────────────────────");
            var vowels = new Queue<char>();

            // Enqueue each English vowel
            char[] vowelArray = { 'a', 'e', 'i', 'o', 'u' };
            foreach (char v in vowelArray)
            {
                vowels.Enqueue(v);
                Console.WriteLine($"  Enqueued: '{v}' | Queue: [{string.Join(", ", vowels)}]");
            }
            Console.WriteLine();

            // Dequeue one by one (FIFO order preserved)
            Console.WriteLine("  Dequeuing all vowels:");
            while (vowels.Count > 0)
            {
                char v = vowels.Dequeue();
                Console.WriteLine($"    Dequeued: '{v}' | Remaining: [{string.Join(", ", vowels)}]");
            }
            Console.WriteLine();

            // ── 3. ToArray & CopyTo ─────────────────────────────────────
            Console.WriteLine("── ToArray / Enumeration ─────────────────────");
            var numbers = new Queue<int>();
            numbers.Enqueue(10);
            numbers.Enqueue(20);
            numbers.Enqueue(30);
            numbers.Enqueue(40);

            // Convert queue to array (preserves order)
            int[] arr = numbers.ToArray();
            Console.WriteLine($"  Queue: {string.Join(", ", numbers)}");
            Console.WriteLine($"  Array: {string.Join(", ", arr)}");

            // Enumerate without modifying the queue
            Console.WriteLine("  Enumerating (queue unchanged):");
            foreach (int n in numbers)
            {
                Console.WriteLine($"    {n}");
            }
            Console.WriteLine();

            // ── 4. TryDequeue & TryPeek (.NET 6+) ──────────────────────
            Console.WriteLine("── Safe Dequeue / Peek ───────────────────────");
            Console.WriteLine($"  TryDequeue on non-empty queue:");
            if (numbers.Count > 0)
            {
                int val = numbers.Dequeue();
                Console.WriteLine($"    Dequeued: {val}");
            }

            // Safely checking before Dequeue avoids InvalidOperationException
            var emptyQueue = new Queue<string>();
            if (emptyQueue.Count > 0)
            {
                emptyQueue.Dequeue(); // won't execute
            }
            else
            {
                Console.WriteLine("    (Queue is empty — skipping Dequeue)");
            }

            Console.WriteLine("\n═══════════════════════════════════════════════");
            Console.WriteLine("  Complexity: Enqueue O(1) | Dequeue O(1) | Peek O(1)");
            Console.WriteLine("  ToArray O(n) | Contains O(n)");
            Console.WriteLine("═══════════════════════════════════════════════");
        }
    }
}