Word Ladder

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.

A transformation sequence is valid when:

• Every adjacent pair of words differs by exactly one letter.
• Every intermediate word (including endWord) must exist in wordList.
• beginWord does not need to be in wordList.

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Example 1

beginWord = "hit"
endWord   = "cog"
wordList  = ["hot","dot","dog","lot","log","cog"]

Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"]
Output: 5
Explanation: "hit" -> "hot" -> "dot" -> "dog" -> "cog" is the shortest path, counting 5 words.

Example 2

beginWord = "hit"
endWord   = "cog"
wordList  = ["hot","dot","dog","lot","log"]

Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log"]
Output: 0
Explanation: "cog" is not in wordList, so no valid sequence exists.

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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