NonRepeatingString, C#

NonRepeatingString

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //nonRepeating("abcab"); // should return 'c'
            //nonRepeating("abab"); // should return null
            //nonRepeating("aabbbc"); // should return 'c'
            //nonRepeating("aabbdbc"); // should return 'd'
            Console.WriteLine(nonRepeating("aabbdbc"));
        }
        
        private static string nonRepeating(string input){
           if( input == null ) return null;
           var charCount = new Dictionary<char, int>();
            var charArray = input.ToCharArray();
            for ( int i = 0; i < charArray.Length; i++ ) {
                if( charCount.ContainsKey(charArray[i]) ) 
                      charCount[charArray[i]] += 1;
                else
                      charCount[charArray[i]] = 1; 
            } 
            
            foreach ( char c in input ) {
              if (charCount[c] == 1) return c.ToString();
            }
        
          return null;
        }
    }
}


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