一个C#面试问题，要求是将字符串中重复字符从第二个开始都去掉，空格除外。然后显示的时候，从后往前显示。

因为C#的code，感觉实现这个还是比较容易，只是SB。所以，在面试时候，以为不会这么容易，所以，我先试着用了Dictionary去实现，发现有困难。然后改回来用StringBuilder实现，这个因为比较简单了，总以为这样会有问题。所以不知道这样是不是对的。

结果当然很不理想，我准备后面学习一下C++，看看这个题目用C++实现有什么值得注意的地方。还是有什么精华所在。

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

namespace ConsoleApplication9
{
    class Program
    {
        public static void DeDupAndReverse(string str)
        {
            char blank = ' ';
            StringBuilder sbDeDup = new StringBuilder();/// SB which will be de dupped.
            StringBuilder sbReverse = new StringBuilder(); /// sb which will be used for reverse

            for (int index = ; index < str.Length; index++)
            {
                if ((!((sbDeDup.ToString()).Contains(str[index])) || (str[index].Equals(blank)))) /// chars contailed in the SB will be dedupped except for blank spance
                {
                    sbDeDup.Append(str[index]);
                }
                else
                {
                    continue;
                }
            }

            for (int index = sbDeDup.Length -; index >= ; index--)
            {
                sbReverse = sbReverse.Append(sbDeDup[index]);   //reverse the sb
            }

            for (int index = ; index < sbReverse.Length; index++)
            {
                System.Console.Write(sbReverse[index]); /// write the SB to the console
            }
        }

        public static void Main(string[] args)
        {
            string str = "Hello World"; /// one test string.

            DeDupAndReverse(str); /// call the method to run a test.

            Console.ReadLine();
        }
    }
}

吸收了一些人的评论，此处没有写注释。更新code如下：

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

namespace ConsoleApplication9
{
    class Program
    {
          public static string ProcessString(string input)
          {
             StringBuilder sb = new StringBuilder();
              HashSet<char> hashSet=new HashSet<char>();
              foreach (var c in input)
             {
                  if (c == ' ')
                  {
                      sb.Append(c);
                 }
                 if (hashSet.Contains(c)==false)
                 {
                    sb.Append(c);
                    hashSet.Add(c);
                 }
             }
             return sb.ToString();
          }

          public static string DeDeup_Reverse(string toBeProcessed)
          {
              string toBeReversed = ProcessString(toBeProcessed);
              StringBuilder sbReversed = new StringBuilder(toBeReversed.Length);

              for (int i = toBeReversed.Length - ; i >= ; i--)
              {
                  sbReversed.Append(toBeReversed[i]);
              }

              return sbReversed.ToString();
          }

        public static void Main(string[] args)
        {
            string str = "Hello World, hello world";

            Console.WriteLine(DeDeup_Reverse(str));

            Console.ReadLine();
        }
    }
}