title: generate all n bit binary numbers in a fastest way possible
body: <p>How do I generate all possible combinations of n-bit strings? I need to generate all combinations of 20-bit strings in a fastest way possible. (my current implementation is done with bitwise AND and right shift operation, but I am looking for a faster technique).</p>
  
  <p>I need to store the bit-strings in an array (or list) for the corresponding decimal numbers, like --</p>
  
  <p><code>0 --&gt; 0 0 0</code></p>
  
  <p><code>1 --&gt; 0 0 1</code></p>
  
  <p><code>2 --&gt; 0 1 0</code> ... etc.</p>
  
  <p>any idea?</p>