title: Identify characters that do (not) appear in a string
body: <p>Let's have the following string (in Python):</p>
  
  <pre><code>str = "There are times when your best efforts are not good enough"
  </code></pre>
  
  <p>Now, is there an efficient way to find the English alphabets (or even the special characters) that didn't appear in that string?</p>
  
  <p>For example, in that string, the alphabets that didn't appear are: <code>c, j, k, l, p, q, v, x, z</code></p>
  
  <p>Thanks.</p>