Given a list of words, find the longest one that's "complete" — every one of its prefixes, from length 1 up to its own full length, must also appear somewhere in the list. If several words tie for longest, return the alphabetically smallest of them; if not a single word qualifies, return the literal string "None". This is the same prefix-chain idea as building a word one character at a time: mark every trie node where some inserted word actually ends, then only ever walk into a child whose own node carries that mark. Any real node the walk reaches this way sits at the end of an unbroken chain of complete words. The one difference from a plain "longest buildable word" search is the empty starting point — the root itself is never a real candidate, so if the walk can't step into even a single child, nothing was complete and the answer falls back to "None" rather than an empty string.