String operations using C#.Net

/*
Following are the methods in C#.Net on String for some of the interviews
1. Reverse a string
2. Remove duplicate words in a give string
3. Remove duplicate chars
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;
namespace Interview
{
  class StringOperations
  {
    /*Reverse the string -- Method 1*/
    public string ReverseString(string inputString)
    {
      StringBuilder sbTemp = new StringBuilder("");
      /*Check either the string is empty or null*/
      if (String.IsNullOrEmpty(inputString))
      {
         return sbTemp.ToString();
      }
      /* If it is a valid string take a single charcter from last till start of the
         string*/
      for (int i = inputString.Length - 1; i >= 0; i--)
      {
        sbTemp.Append(inputString.Substring(i, 1));
      }
      return sbTemp.ToString();
    }

    /*Remove duplicate words in a give string.*/
    public string RemoveDuplicateWords(string stringToRemoveDuplicates)
    {
      if (String.IsNullOrEmpty(stringToRemoveDuplicates))
      {
        return "";
      }
      ArrayList alist = new ArrayList();
      foreach (string word in stringToRemoveDuplicates.Split(' '))
      {
        if (!alist.Contains(word))
        {
          alist.Add(word);
        }
      }
      return string.Join(" ", (string[])alist.ToArray(typeof(string)));
    }

     /*Remove duplicate chars*/
     public string RemoveDuplicateChars(string removeDups)
     {
       StringBuilder sb = new StringBuilder(string.Empty);
       foreach (char c in removeDups.ToCharArray())
       {
        if (sb.ToString().IndexOf(c) == -1)
        {
          sb.Append(c);
        }
       }
       return sb.ToString();
      }
  }
}