Task: Encode the alphabet as a binary tree using the frequencies of letters in the given text.

• For each character in the text calculate its number of occurrences.
  eg. for string acdc it would be [a -> 1], [c -> 2], [d -> 1]
• For each character and its frequency create a one-node tree.
• Take two trees T1 and T2 with the lowest frequencies and merge them into a larger tree T12
  (T1 should become the left sub-tree and T2 the right subtree).
• Repeat the previous step until there is only 1 tree left.

That last tree represents the created encoding. For example, given text ababacccccc you should get: