You get a start word begin_word, a target word end_word and a list of allowed words word_list. All the words have the same length and use lowercase letters. A ladder goes from begin_word to end_word one word at a time, and each word differs from the word before it in exactly one position. begin_word may be missing from the list, but every later word on the ladder must come from it. The list may also hold a word twice, or hold begin_word.

Count the words on the shortest ladder, with begin_word and end_word both included, and return that count. If no ladder exists, for example because end_word is not in the list, return 0.