Find the first un-repeated character in a string

Question

What is the quickest way to find the first character which only appears once in a string?

Answer 1

The following code is in C# with complexity of n.

using System;
using System.Linq;
using System.Text;

namespace SomethingDigital
{
    class FirstNonRepeatingChar
    {
        public static void Main()
        {
            String input = "geeksforgeeksandgeeksquizfor";
            char[] str = input.ToCharArray();

            bool[] b = new bool[256];
            String unique1 = "";
            String unique2 = "";

            foreach (char ch in str)
            {
                if (!unique1.Contains(ch))
                {
                    unique1 = unique1 + ch;
                    unique2 = unique2 + ch;
                }
                else
                {
                    unique2 = unique2.Replace(ch.ToString(), "");
                }
            }
            if (unique2 != "")
            {
                Console.WriteLine(unique2[0].ToString());
                Console.ReadLine();
            }
            else
            {
                Console.WriteLine("No non repeated string");
                Console.ReadLine();
            }
        }
    }
}