Solution1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace cSharpSolutions
{
    internal class Solution1
    {
        // Basic approach: loops through string
        public static char? FirstNonRepeatingChar(string input)
        {
            for (int i = 0; i < input.Length; i++)
            {
                bool repeated = false;
                for (int j = 0; j < input.Length; j++)
                {
                    if (i != j && input[i] == input[j])
                    {
                        repeated = true;
                        break;
                    }
                }
                if (!repeated)
                    return input[i];
            }
            return null; // no non-repeating character found
        }

        // Optimized approach using Dictionary
        public static char? FirstNonRepeatingCharOptimized(string input)
        {
            var countDict = new Dictionary<char, int>();

            foreach (var c in input)
            {
                if (countDict.ContainsKey(c))
                    countDict[c]++;
                else
                    countDict[c] = 1;
            }

            foreach (var c in input)
            {
                if (countDict[c] == 1)
                    return c;
            }

            return null;
        }

        // Run method for console testing
        public static void Run()
        {
            Console.WriteLine("=== Solution 1: First Non-Repeating Character ===");

            string[] testCases =
            {
                "aabccdeff",   // Expected: b
                "aabbcc",      // Expected: null
                "abcdef",      // Expected: a
                "mmalawi",     // Expected: l
                "zzxxyy",      // Expected: null
                "lacsonimran",  // Expected: l
                "llaccssooniimmran"  // Expected: r
            };

            foreach (var test in testCases)
            {
                var basic = FirstNonRepeatingChar(test);
                var optimized = FirstNonRepeatingCharOptimized(test);

                Console.WriteLine($"\nInput: \"{test}\"");
                Console.WriteLine($"  Basic: {(basic != null ? basic.ToString() : "null")}");
                Console.WriteLine($"  Optimized: {(optimized != null ? optimized.ToString() : "null")}");
            }

            Console.WriteLine("\nSolution 1 complete");
        }
    }
}