Program.cs

//Question 1: Write an funciton that finds the largest and smallest numbers in an array 
using System.Text;
using System.Text.RegularExpressions;

static (int min, int max) FindMinMax(int[] numbers)
{
    if (numbers == null || numbers.Length == 0)
        throw new ArgumentException("Array cannot be null or empty.");

    int min = numbers[0];
    int max = numbers[0];

    for (int i = 1; i < numbers.Length; i++)
    {
        if (numbers[i] < min)
            min = numbers[i];

        if (numbers[i] > max)
            max = numbers[i];
    }

    return (min, max);
}
//Question 2: Write a function that removes duplicate characters from string. Provide at least 3 solutions.
//Which is best in your opinion? Why?
//2A
static string RemoveDuplicates_HashSet(string input)
{
    if (string.IsNullOrEmpty(input)) return input;
    var seen = new HashSet<char>();
    var result = new StringBuilder();
    foreach (char c in input)
    {
        // returns false if exists
        if (seen.Add(c)) 
        {
            result.Append(c);
        }
    }
    return result.ToString();
}
//2B
static string RemoveDuplicates_Linq(string input)
{
    if (string.IsNullOrEmpty(input)) return input;
    return new string(input.Distinct().ToArray());
}
//2C
static string RemoveDuplicates_NoExtraSpace(string input)
{
    if (string.IsNullOrEmpty(input)) return input;
    var result = new StringBuilder();
    for (int i = 0; i < input.Length; i++)
    {
        bool isDuplicate = false;
        for (int j = 0; j < i; j++)
        {
            if (input[i] == input[j])
            {
                isDuplicate = true;
                break;
            }
        }
        if (!isDuplicate)
        {
            result.Append(input[i]);
        }
    }
    return result.ToString();
}
//2a is best becuase O(n) time, Keeps original order, Clean and efficient

//Question 3: Write a function that checks if 2 strings are anagrams.
static bool isAnagram(string s1, string s2)
{
    //error handling
    if (s1 == null || s2 == null) return false;
    // If lengths are different, they cannot be anagrams
    if (s1.Length != s2.Length) return false;
    // Dictionary to store character frequency counts
    var counts = new Dictionary<char, int>();
    // Count each character in the first string
    foreach (char c in s1.ToLower())
    {
        // If character not yet seen, initialize count
        if (!counts.ContainsKey(c))
            counts[c] = 0;
        // Increment count for this character
        counts[c]++;
    }
    // Subtract counts using the second string
    foreach (char c in s2.ToLower())
    {
        // If character not found, strings are not anagrams
        if (!counts.ContainsKey(c))
            return false;
        // Decrease count for this characte
        counts[c]--;
        // If count goes negative, s2 has extra occurrences
        if (counts[c] < 0)
            return false;
    }
    return true;
}
//Question 4: Write a RegEx to match an Australian mobile phone
static bool IsValidAustralianMobile(string phoneNumber)
{
    if (string.IsNullOrWhiteSpace(phoneNumber))
        return false;
    // Matches:
    // 04XXXXXXXX
    // +614XXXXXXXX
    // with optional spaces or hyphens

    // ^                Start of string
    // (?:\+61|0)        Match either "+61" (international format) OR "0" (local format)
    // 4                 Mobile numbers must start with '4' after prefix
    // (?:[\s-]?\d){8}  Match 8 digits, each optionally preceded by a space or hyphen
    // $                 End of string
    string pattern = @"^(?:\+61|0)4(?:[\s-]?\d){8}$";
    return Regex.IsMatch(phoneNumber, pattern);
}

Console.WriteLine("Hello, World!");

//Question 1: Example
var result = FindMinMax(new[] { 123, 534, 3, 9, 275 });
Console.WriteLine($"Min: {result.min}, Max: {result.max}");
//Question 2: Example
//2A
Console.WriteLine($"Input String for removing duplicate characters is 'Hello'");
var inputString = "hello";
var string1 = RemoveDuplicates_HashSet(inputString);
Console.WriteLine($"RemoveDuplicates_HashSet");
Console.WriteLine($"{string1}");
//2B
var string2 = RemoveDuplicates_Linq(inputString);
Console.WriteLine($"RemoveDuplicates_Linq");
Console.WriteLine($"{string2}");
//2C
var string3 = RemoveDuplicates_NoExtraSpace(inputString);
Console.WriteLine($"RemoveDuplicates_NoExtraSpace");
Console.WriteLine($"{string3}");
//Question 3: Example
//true
var anagramString = isAnagram("note","tone");
Console.WriteLine($"Check Anagram for Strings 'note' and 'tone'");
Console.WriteLine($"{anagramString}");
//false
var nonAnagramString = isAnagram("hello","world");
Console.WriteLine($"Check Anagram for Strings 'hello' and 'world'");
Console.WriteLine($"{nonAnagramString}");
//Question 4: Example
Console.WriteLine($"Check if Australian phone: +61412 345 678");
Console.WriteLine(IsValidAustralianMobile("+61412 345 678"));
Console.WriteLine($"Check if Australian phone: 0412 345 678");
Console.WriteLine(IsValidAustralianMobile("0412 345 678"));
Console.WriteLine($"Check if Australian phone: +61312 345 678");
Console.WriteLine(IsValidAustralianMobile("+61312 345 678"));
Console.WriteLine($"Check if Australian phone: 0312 345 678");
Console.WriteLine(IsValidAustralianMobile("0312 345 678"));