title: List of all unique characters in a string?
body: <p>I want to append characters to a string, but want to make sure all the letters in the final list are <strong>unique</strong>.</p>
  
  <p>Example: <code>"aaabcabccd"</code> → <code>"abcd"</code></p>
  
  <p>Now of course I have two solutions in my mind. One is using a <code>list</code> that will map the characters with their ASCII codes. So whenever I encounter a letter it will set the index to <code>True</code>. Afterwards I will scan the list and append all the ones that were set. It will have a time complexity of <strong>O(n)</strong>.</p>
  
  <p>Another solution would be using a <code>dict</code> and following the same procedure. After mapping every char, I will do the operation for each key in the dictionary. This will have a <strong>linear</strong> running time as well.</p>
  
  <p>Since I am a Python newbie, I was wondering which would be more space efficient. Which one could be implemented more efficiently?</p>
  
  <p><strong>PS</strong>: Order is <strong><em>not</em></strong> important while creating the list.</p>