Graph — Word Ladder

solve(beginWord, endWord, wordList) — transform beginWord into endWord by
changing one letter at a time; every intermediate word must be in wordList
(beginWord itself does not have to be). Return the number of words in the
shortest transformation sequence, counting both ends, or 0 if there is none.

Words are nodes, and two words are connected when they differ by exactly one
letter, so this is a BFS for the shortest path in an implicit graph.

Examples

solve("hit", "cog", ["hot","dot","dog","lot","log","cog"])  → 5   // hit→hot→dot→dog→cog
solve("hit", "cog", ["hot","dot","dog","lot","log"])        → 0   // "cog" is missing
solve("a", "c", ["a","b","c"])                              → 2
solve("red", "tax", ["ted","tex","red","tax","tad","den","rex","pee"]) → 4

All words have the same length and are lowercase. Generating neighbours by trying
each of the 26 letters at every position is much cheaper than comparing every pair.