Shortest Word Transformation Sequence

You are given two different words, beginWord and endWord, and a dictionary wordList. A transformation changes exactly one letter, and every transformed word, including endWord, must appear in the dictionary.
Return the number of words in the shortest valid transformation sequence from beginWord through endWord, counting both endpoints. Return 0 when no valid sequence exists.

Function
ladderLength(beginWord: String, endWord: String, wordList: String[]) → int

Examples
Example 1
beginWord = "hit"
endWord = "cog"
wordList = ["hot","dot","dog","lot","log","cog"]
return = 5
One shortest sequence is hit → hot → dot → dog → cog, which contains five words.
Example 2
beginWord = "hit"
endWord = "cog"
wordList = ["hot","dot","dog","lot","log"]
return = 0
The endpoint is absent from the dictionary, so no valid transformation sequence exists.
Example 3
beginWord = "a"
endWord = "c"
wordList = ["a","b","c"]
return = 2
Changing a directly to c uses two words.

Constraints
1 <= beginWord.length == endWord.length <= 10.
1 <= wordList.length <= 5000.
All words have the same length and contain only lowercase English letters.
beginWord and endWord are different.
The dictionary contains no duplicate words.

The trick: shortest path with equal-cost edges means BFS, not DFS. Put beginWord in a queue with a level of 1. For each word, try swapping each position with a through z, and check whether the new word is in the set. If it is, enqueue it and remove it from the set so you never visit it twice. Return the level when you pop endWord. Pitfalls: forgetting that endWord must be in the dictionary (Example 2 returns 0), counting edges instead of words (Example 3 returns 2, not 1), and comparing every pair of words, which is slow with 5000 words. Generating neighbors costs about 10 x 26 lookups per word, which is far cheaper. Note that beginWord doesn't need to be in the list. If the search runs dry, return 0. StealthCoder is your hedge if the live OA clock rattles you and the neighbor loop won't come out right.

StealthCoder is the hedge for the one pattern you didn't drill. It runs invisibly during the screen share.