Word Ladder – Solution & Complexity

def ladder_length(beginWord: str, endWord: str, wordList: list[str]) -> int:
    word_set = set(wordList)
    if endWord not in word_set:
        return 0

    queue = [(beginWord, 1)]
    head = 0
    if beginWord in word_set:
        word_set.remove(beginWord)

    while head < len(queue):
        word, distance = queue[head]
        head += 1
        if word == endWord:
            return distance

        letters = list(word)
        for i in range(len(letters)):
            original = letters[i]
            for code in range(ord('a'), ord('z') + 1):
                char = chr(code)
                if char == original:
                    continue
                letters[i] = char
                candidate = ''.join(letters)
                if candidate in word_set:
                    word_set.remove(candidate)
                    queue.append((candidate, distance + 1))
            letters[i] = original

    return 0