title: Converting hex to binary with leading zeros
body: <p>I am following the below method</p>
  
  <pre><code>new Biginteger(str,16).toString(2);
  </code></pre>
  
  <p>It works really well, but it removes leading zeros. I need output in 64 bits if input string is "3031323334353637"</p>
  
  <p>But it returns 62 characters. I can use a for loop to do that. Is there any other way to do that without loop?</p>
  
  <p><strong>Goal:</strong> Converting hex to binary with leading zeros</p>