title: Determing if string has all unique characters
body: <p>How can I do this without using arrays?
  I have this implementation, but I need to do it without using arrays, bit operations and any other C/C++ libraries.</p>
  
  <pre><code>bool IsCharDuplication(string s) {
    bool result = false;
    bool controlArray[256];
    for (int i = 0; i &lt; s.length(); i++) {
      int val = s[i];
      if (controlArray[val] == true) {
        result = true;
        break;
      }
      controlArray[val] = true;
    }
    return result;
  }
  </code></pre>