02_HashSet.cs

using System;
using System.Collections.Generic;

namespace DataStructures
{
    /// <summary>
    /// Demonstrates the HashSet<T> collection in C#.
    /// A HashSet stores unique elements and provides O(1) average-time
    /// add, remove, and lookup operations. It supports mathematical set
    /// operations such as union, intersection, and difference.
    /// 
    /// Common use cases: removing duplicates, membership testing,
    /// set algebra (union, intersection, difference).
    /// </summary>
    public class HashSetExamples
    {
        // ─── Entry Point ───────────────────────────────────────────────
        public static void Main()
        {
            Console.WriteLine("═══════════════════════════════════════════════");
            Console.WriteLine("           C# HashSet<T> Examples              ");
            Console.WriteLine("═══════════════════════════════════════════════\n");

            // ── 1. Basic Operations with Vowels ─────────────────────────
            Console.WriteLine("── English Vowels ─────────────────────────────");
            var vowels = new HashSet<char> { 'a', 'e', 'i', 'o', 'u' };

            Console.WriteLine($"  Initial vowels: {string.Join(", ", vowels)}");
            Console.WriteLine($"  Count         : {vowels.Count}");
            Console.WriteLine();

            // ── 2. Add & Remove ─────────────────────────────────────────
            Console.WriteLine("── Add / Remove Operations ───────────────────");

            // Add returns true if the element was added (not already present)
            bool addedUpper = vowels.Add('A');
            Console.WriteLine($"  Add('A') → added = {addedUpper}");

            bool addedDuplicate = vowels.Add('a');
            Console.WriteLine($"  Add('a') → added = {addedDuplicate}  (duplicate — ignored)");

            // Remove an element
            bool removed = vowels.Remove('A');
            Console.WriteLine($"  Remove('A') → removed = {removed}");
            Console.WriteLine($"  Current vowels: {string.Join(", ", vowels)}");
            Console.WriteLine();

            // ── 3. Set Operations ───────────────────────────────────────
            Console.WriteLine("── Set Operations ─────────────────────────────");

            var setA = new HashSet<char> { 'a', 'e', 'i', 'o', 'u' };
            var setB = new HashSet<char> { 'i', 'o', 'u', 'x', 'y' };

            Console.WriteLine($"  Set A (vowels)   : {string.Join(", ", setA)}");
            Console.WriteLine($"  Set B (mixed)    : {string.Join(", ", setB)}");

            // UnionWith — adds all elements from setB into setA
            var union = new HashSet<char>(setA);
            union.UnionWith(setB);
            Console.WriteLine($"  Union (A ∪ B)    : {string.Join(", ", union)}");

            // IntersectWith — keeps only elements present in both sets
            var intersect = new HashSet<char>(setA);
            intersect.IntersectWith(setB);
            Console.WriteLine($"  Intersect (A ∩ B): {string.Join(", ", intersect)}");

            // ExceptWith — removes elements from setA that are in setB
            var difference = new HashSet<char>(setA);
            difference.ExceptWith(setB);
            Console.WriteLine($"  Difference (A−B) : {string.Join(", ", difference)}");

            // SymmetricExceptWith — elements in A or B but not both (XOR)
            var symmetricDiff = new HashSet<char>(setA);
            symmetricDiff.SymmetricExceptWith(setB);
            Console.WriteLine($"  SymDiff (A △ B)  : {string.Join(", ", symmetricDiff)}");
            Console.WriteLine();

            // ── 4. Membership & Overlap Checks ──────────────────────────
            Console.WriteLine("── Membership / Comparison ───────────────────");
            Console.WriteLine($"  setA.Contains('e')         : {setA.Contains('e')}");
            Console.WriteLine($"  setA.Contains('z')         : {setA.Contains('z')}");
            Console.WriteLine($"  setA.Overlaps(setB)        : {setA.Overlaps(setB)}");
            Console.WriteLine($"  setA.IsSubsetOf(setB)      : {setA.IsSubsetOf(setB)}");
            Console.WriteLine($"  intersect.IsSubsetOf(setA) : {intersect.IsSubsetOf(setA)}");
            Console.WriteLine();

            // ── 5. Removing Duplicates from a List ──────────────────────
            Console.WriteLine("── Removing Duplicates ────────────────────────");
            var numbers = new List<int> { 1, 3, 5, 3, 7, 1, 9, 5, 11, 3 };
            Console.WriteLine($"  Original list: {string.Join(", ", numbers)}");

            var uniqueNumbers = new HashSet<int>(numbers);
            Console.WriteLine($"  Unique values: {string.Join(", ", uniqueNumbers)}");
            Console.WriteLine($"  Duplicates removed: {numbers.Count - uniqueNumbers.Count}");

            Console.WriteLine("\n═══════════════════════════════════════════════");
            Console.WriteLine("  Complexity: Add O(1) | Remove O(1) | Contains O(1)");
            Console.WriteLine("  Set operations: O(n) where n = size of smaller set");
            Console.WriteLine("═══════════════════════════════════════════════");
        }
    }
}