Word Ladder Length

Given begin, end and words (equal-length lowercase ASCII words), change one character at a time so each word AFTER begin belongs to words. Return the number of words in the shortest chain, including begin and end, or 0 if impossible. begin need not belong to words. If begin == end, return 1 even if absent from words. Duplicate dictionary entries do not matter. At most 200 words of length at most 8. Examples (JSON; null means Python None): Input: {"begin": "hit", "end": "cog", "words": ["hot", "dot", "dog", "lot", "log", "cog"]} Output: 5 Input: {"begin": "hit", "end": "cog", "words": ["hot", "dot", "dog"]} Output: 0

def word_ladder(begin: str, end: str, words: list[str]) -> int:
    """Count words along the shortest valid transformation chain."""
    pass