Word ladder

class Solution:
    def ladderLength(self, beginWord: str, endWord: str, wordList: List[str]) -> int:
        wordSet = set(wordList)
        if len(wordSet)== 0 or endWord not in wordSet:
            return 0
        mapping = {beginWord:1}
        queue = deque([beginWord]) 
        while queue:
            word = queue.popleft()
            path = mapping[word]
            for i in range(len(word)):
                word_list = list(word)
                for j in range(26):
                    word_list[i] = chr(ord('a')+j)
                    newWord = "".join(word_list)
                    if newWord == endWord:
                        return path+1
                    if newWord in wordSet and newWord not in mapping:
                        mapping[newWord] = path+1
                        queue.append(newWord)                      
        return 0

Word Ladder

Difficulty: Hard

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.