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word ladder换壳题。BFS解决。

class Solution:    def minMutation(self, start: str, end: str, bank: List[str]) -> int:        wordset = set(bank)        #print(wordset)        wordset.add(start)        meet = set()        if end not in wordset:return -1        res = 0        que = collections.deque()        que.append(start)        letters = 'ACTG'        while que:            size = len(que)            for i in range(size):                curword = que.popleft()                meet.add(curword)                for j in range(8):                    for c in letters:                        curnewword = curword[:j] + c + curword[j+1:]                        if curnewword not in wordset:                            continue                        if curnewword not in meet:                            if curnewword == end:return res+1                            else:que.append(curnewword)            res += 1        return -1

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