148ms. Beat 100% Python submissions. Two END BFS + PathMap

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    Sharing my solution based on two-end BFS and PathMap. The basic ideas are:

    1. Using words in either beginSet or endSet depends on length.
    2. Loop through all new words with one char difference and update PathMap and nextlvl.
    3. Going through PathMap and build solution.


    import string
    from collections import defaultdict
    class Solution(object):
        def findLadders(self, beginWord, endWord, wordList):
            :type beginWord: str
            :type endWord: str
            :type wordList: List[str]
            :rtype: List[List[str]]
            wordList = set(wordList)
            if endWord not in wordList: return []
            if beginWord in wordList: wordList.remove(beginWord)
            if endWord in wordList: wordList.remove(endWord)
            beginSet, endSet = set([beginWord]), set([endWord])
            ansFound = False
            mp = defaultdict(list)
            while beginSet and endSet:
                if len(beginSet) < len(endSet):
                    isBeginSmall, small, big = True, beginSet, endSet
                    isBeginSmall, small, big = False, endSet, beginSet
                nextlvl = set([])
                for word in small:
                    for i in xrange(len(word)):
                        for char in string.lowercase:
                            newword = word[:i] + char + word[i+1:]
                            if newword in big: or newword in wordList:
                                if newword in big: 
                                    ansFound = True
                                if isBeginSmall: mp[word].append(newword)
                                else: mp[newword].append(word)
                if ansFound: break
                for word in nextlvl:
                if isBeginSmall: beginSet = nextlvl
                else: endSet = nextlvl
            if not ansFound: return []
            res = [[beginWord]]
            while res[0][-1] != endWord:
                tmp = res
                res = []
                for line in tmp:
                    for word in mp[line[-1]]:
                        res.append(line + [word])
            return res

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