title: Create tree of characters
body: <p>I have to create a tree in this way: take a string, using the first characters as root. The left/right son will be the tree create whit the substring containing minor/major characters (using alphabetical order) of the root and so on. My problem is the recursive call that does not work. Any suggestion? made myself clear? I hope so...
  Example:</p>
  
  <pre><code>input: hello
  output:    
      h
     / \
    e   l
       / \
      l   o
  </code></pre>
  
  <p>any advice on how to improve is accepted. Thank you!</p>