title: How to get distinct characters?
body: <p>I have a code like</p>
  
  <pre><code>string code = "AABBDDCCRRFF";
  </code></pre>
  
  <p>In this code, I want to retrieve only distinct characters</p>
  
  <p>The Output should be like:</p>
  
  <pre><code>ANS: ABDCRF
  </code></pre>