题目：在一个字符串中找到第一个只出现一次的字符。如输入abaccdeff，则输出b。

题目：在一个字符串中找到第一个只出现一次的字符。如输入abaccdeff，则输出b。  
分析：这道题是2006年google的一道笔试题。

C# codes as below:

using System;

using System.Collections.Generic;

namespace ConsoleApp

{

    class RunClass

    {

        static void Main()

        {

            char i = new Helper().Print("2233") ?? ' ';

            Console.WriteLine(i);

            Console.ReadLine();

        }

    }

    class Helper

    {

        public char? Print(string str)

        {

            if (str.Length == 0 || str == null)

                return null;

            bool[] marks = new bool[str.Length];

            for (int i = 0; i < str.Length; i++)

            {

                if (marks[i] != true)

                {

                    marks[i] = true;

                    int count = 0;

                    for (int j = i; j < str.Length; j++)

                    {

                        if (str[j] == str[i])

                        {

                            count++;

                            marks[j] = true;

                        }                    

                    }

                    if (count == 1)

                    {

                        return str[i];

                    }

                }

            }

            return null;

        }

    }

}