title: What does 256 means for 128 unique characters in ascii table
body: <p>If I need to check a string has unique characters, I understand if we are considering characters in Ascii table, then there will 128 of them. </p>
  
  <p>However, why do we need to make a boolean array of size 256 to hold 128 characters to check if element existed at least once in a string? Shouldn't a boolean array of size 128 sufficient?</p>
  
  <p>Here's a quote from from the book "Cracking the Coding Interview":</p>
  
  <pre><code>if (str.length() &gt; 128) return false;
  boolean[] char_set = new boolean[256]; //which is strange since it clearly says over 128 its false
  </code></pre>
  
  <p>.....</p>