title: Write a function that compares two strings and returns a third string containing only the letters that appear in both
body: <p>I got this homework. And have solved it in following way. I need your comments whether it is a good approach or I need to use any other data sturcture to solve it in better way.</p>
  
  <pre><code>    public string ReturnCommon(string firstString, string scndString)
      {
          StringBuilder newStb = new StringBuilder();
          if (firstString != null &amp;&amp; scndString != null)
          {
              foreach (char ichar in firstString)
              {
                  if (!newStb.ToString().Contains(ichar) &amp;&amp; scndString.Contains(ichar))
                      newStb.Append(ichar);
              }
          }
          return newStb.ToString();
      }
  </code></pre>