LeetCode 127. Word Ladder



Given two words (beginWord and endWord), and a dictionary’s word list, find the length of shortest transformation sequence from beginWord to endWord, such that:

  1. Only one letter can be changed at a time.
  2. Each transformed word must exist in the word list.


  • Return 0 if there is no such transformation sequence.
  • All words have the same length.
  • All words contain only lowercase alphabetic characters.
  • You may assume no duplicates in the word list.
  • You may assume beginWord and endWord are non-empty and are not the same.

Example 1:

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

Output: 5

Explanation: As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
return its length 5.

Example 2:

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

Output: 0

Explanation: The endWord "cog" is not in wordList, therefore no possible transformation.



Python Solution

class Solution:
    def ladderLength(self, beginWord: str, endWord: str, wordList: List[str]) -> int:
        wordset = set(wordList)
        queue = deque()
        queue.append((beginWord, 1))
        word_length = len(beginWord)
        while queue:
            word, step = queue.popleft()
            if word == endWord:
                return step
                for i in range(0, word_length):
                    for c in "abcdefghijklmnopqrstuvwxyz":
                        new_word = word[:i] + c + word[i+1:]
                        if new_word in wordset:
                            queue.append((new_word, step + 1))
        return 0
  • Time Complexity: ~N
  • Space Complexity: ~1

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