Given two words (beginWord and endWord), and a dictionary's word list, find the length of shortest transformation sequence from beginWord to endWord, such that:


       Only one letter can be changed at a time.
       Each transformed word must exist in the word list. Note that beginWord is not a transformed word.


Note:


       Return 0 if there is no such transformation sequence.
       All words have the same length.
       All words contain only lowercase alphabetic characters.
       You may assume no duplicates in the word list.
       You may assume beginWord and endWord are non-empty and are not the same.


Example 1:


Input:
beginWord = "hit",
endWord = "cog",
wordList = ["hot","dot","dog","lot","log","cog"]

Output: 5

Explanation: As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
return its length 5.


Example 2:


Input:
beginWord = "hit"
endWord = "cog"
wordList = ["hot","dot","dog","lot","log"]

Output: 0

Explanation: The endWord "cog" is not in wordList, therefore no possible transformation.

Difficulty: interview

Starter code:

class Solution:
    def ladderLength(self, beginWord: str, endWord: str, wordList: List[str]) -> int:
        

Solutions:
class Solution:
     def ladderLength(self, beginWord, endWord, wordList):
 
         wordDict = set(wordList)
         if not endWord in wordDict:
             return 0
 
         visited = set()
 
         beginSet = set()
         beginSet.add(beginWord)
         visited.add(beginWord)
 
         endSet = set()
         endSet.add(endWord)
         visited.add(endWord)
 
         lenWord = len(beginWord)
 
         distance = 1
 
         while len(beginSet) > 0 and len(endSet) > 0:
 
             # make sure begin set is smaller than endSet
             if len(beginSet) > len(endSet):
                 beginSet, endSet = endSet, beginSet
 
             # extend begin set
             newSet = set()
             for w in beginSet:
                 for i in range(lenWord):
                     part1 = w[:i]
                     part2 = w[i+1:]
                     for alpha in 'abcdefghijklmnopqrstuvwxyz':
                         target = part1 + alpha + part2
 
                         if target in endSet:
                             return distance + 1
                         elif (not target in visited) and (target in wordDict):
                             newSet.add(target)
                             visited.add(target)
 
             beginSet = newSet
             distance += 1
         return 0

class Solution:
     def ladderLength(self, beginWord, endWord, wordList):
         """
         :type beginWord: str
         :type endWord: str
         :type wordList: List[str]
         :rtype: int
         """             
 
         words = set(wordList)
         if endWord not in words:
             return 0
         
         beginQ, endQ = {beginWord}, {endWord}
         dist = 2
         while beginQ:
             print(('beginQ', beginQ, 'endQ', endQ))
             newq = set()
             for u in beginQ:
                 for i in range(len(u)):
                     for c in 'abcdefghijklmnopqrstuvwxyz':
                         v = u[:i] + c + u[i+1:]
                         if v in words:
                             if v in endQ:
                                 return dist
                             newq.add(v)
                             
             dist += 1            
             beginQ = newq
             if len(beginQ) > len(endQ):
                 beginQ, endQ = endQ, beginQ
                 
             words -= beginQ
         
         return 0
             
 
                         


class Solution():
     def ladderLength(self, beginWord, endWord, wordDict):
         front, back = set([beginWord]), set([endWord])
         wordDict = set(wordDict)
         length = 2
         width = len(beginWord)
         charSet = 'abcdefghijklmnopqrstuvwxyz'
         
         if endWord not in wordDict:
             return 0
         
         while front:
             newFront = set()
             
             for phase in front:
                 for i in range(width):
                     for c in charSet:
                         nw = phase[:i] + c + phase[i+1:]
                         if nw in back:
                             return length
                         if nw in wordDict:
                             newFront.add(nw)
             front = newFront
             
             if len(front) > len(back):
                 front,back = back,front
                 
             wordDict -= wordDict & front
             length += 1
         
         return 0

class Solution:
     def ladderLength(self, beginWord, endWord, wordList):
         """
         :type beginWord: str
         :type endWord: str
         :type wordList: List[str]
         :rtype: int
         """
         letters ='abcdefghijklmnopqrstuvwxyz'
         wordList = set(wordList)
         if endWord not in wordList:
             return 0
         reached = set()
         backreached = set()
         reached.add(beginWord)
         backreached.add(endWord)
         wordList.discard(beginWord)
         wordList.discard(endWord)
         depth = 1
         while not (reached & backreached):
             toAdd = set()
             for word in reached:
                 for i,letter in enumerate(word):
                     for newletter in letters:
                         if newletter != letter:
                             newword = word[:i] + newletter + word[i+1:]
                             if newword in backreached:
                                 return depth + 1
                             elif newword in wordList:
                                 toAdd.add(newword)
                                 wordList.remove(newword)
             if len(toAdd) == 0:
                 return 0
             else:
                 depth += 1
                 reached = toAdd
                 if len(reached) > len(backreached):
                     reached, backreached = backreached, reached
         return depth

class Solution:
     def ladderLength(self, beginWord, endWord, wordList):
         """
         :type beginWord: str
         :type endWord: str
         :type wordList: List[str]
         :rtype: int
         """
         wordList=set(wordList)
         if endWord not in wordList:
             return 0
         beginSet=set()
         endSet=set()
         visited=set()
         res=1
         beginSet.add(beginWord)
         endSet.add(endWord)
         while len(beginSet)!=0 and len(endSet)!=0:
             if len(beginSet)>len(endSet):
                 beginSet,endSet=endSet,beginSet
             tmp=set()
             for word in beginSet:
                 chs=list(word)
                 for i in range(len(chs)):
                     for ch in range(ord('a'),ord('z')+1):
                         old=chs[i]
                         chs[i]=chr(ch)
                         target="".join(chs)
                         if target in endSet:
                             return res+1
                         if target not in visited and target in wordList:
                             tmp.add(target)
                             visited.add(target)
                         chs[i]=old
             beginSet=tmp
             res+=1
         return 0