Problem Statement

A certain children's game involves starting with a word in a particular category.   Each participant in turn says a word, but that word must begin with the final letter of the previous word.   Once a word has been given, it cannot be repeated.   If an opponent cannot give a word in the category, they fall out of the game.

For example, with   "animals"   as the category,

Take the following selection of 70 English Pokemon names   (extracted from   Wikipedia's list of Pokemon)   and generate the/a sequence with the highest possible number of Pokemon names where the subsequent name starts with the final letter of the preceding name. No Pokemon name is to be repeated.

Extra brownie points for dealing with the full list of   646   names.

Let's start with the solution:

(de pokemonChain (File)
   (let Names (make (in File (while (read) (link @))))
      (for Name Names
         (let C (last (chop Name))
            (set Name
               (filter '((Nm) (pre? C Nm)) Names) ) ) )
      (let Res NIL
         (for Name Names
            (let Lst NIL
               (recur (Name Lst)
                  (if (or (memq Name Lst) (not (val (push 'Lst Name))))
                     (when (> (length Lst) (length Res))
                        (setq Res Lst) )
                     (mapc recurse (val Name) (circ Lst)) ) ) ) )
         (flip Res) ) ) )