Word Ladder

A **transformation sequence** from word `beginWord` to word `endWord` using a dictionary `wordList` is a sequence of words `beginWord -> s1 -> s2 -> ... -> sk` such that: - Every adjacent pair of words differs by a single letter. - Every `si` for `1 <= i <= k` is in `wordList`. Note that `beginWord` does not need to be in `wordList`. - `sk == endWord` Given two words, `beginWord` and `endWord`, and a dictionary `wordList`, return _the **number of words** in the **shortest transformation sequence** from_ `beginWord` _to_ `endWord`_, or _`0`_ if no such sequence exists._   **Example 1:** **Input:** beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"] **Output:** 5 **Explanation:** One shortest transformation sequence is "hit" -> "hot" -> "dot" -> "dog" -> cog", which is 5 words long. ``` **Example 2:** **Input:** beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log"] **Output:** 0 **Explanation:** The endWord "cog" is not in wordList, therefore there is no valid transformation sequence. ```   **Constraints:** - `1 <= beginWord.length <= 10` - `endWord.length == beginWord.length` - `1 <= wordList.length <= 5000` - `wordList[i].length == beginWord.length` - `beginWord`, `endWord`, and `wordList[i]` consist of lowercase English letters. - `beginWord != endWord` - All the words in `wordList` are **unique**.