127-word-ladder.py

"""
Problem Link: https://leetcode.com/problems/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.
"""
class Solution:
    def ladderLength(self, beginWord: str, endWord: str, wordList: List[str]) -> int:
        wordList = set(wordList)
        queue = [beginWord]
        count = 0
        
        while queue:
            count += 1
            
            for _ in range(len(queue)):
                word = queue.pop(0)

                if word == endWord:
                    return count

                for i in range(len(word)):
                    for c in 'abcdefghijklmnopqrstuvwxyz':

                        next_word = word[:i] + c + word[i+1:]

                        if next_word in wordList:
                            wordList.remove(next_word)
                            queue.append(next_word)
        return 0