word-ladder.py

# Time:  O(n * d), n is length of string, d is size of dictionary
# Space: O(d)
#
# Given two words (start and end), and a dictionary, find the length of shortest transformation sequence from start to end, such that:
# 
# Only one letter can be changed at a time
# Each intermediate word must exist in the dictionary
# For example,
# 
# Given:
# start = "hit"
# end = "cog"
# dict = ["hot","dot","dog","lot","log"]
# As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
# return its length 5.
# 
# Note:
# Return 0 if there is no such transformation sequence.
# All words have the same length.
# All words contain only lowercase alphabetic characters.
#

# BFS
class Solution(object):
    def ladderLength(self, beginWord, endWord, wordList):
        """
        :type beginWord: str
        :type endWord: str
        :type wordList: List[str]
        :rtype: int
        """
        distance, cur, visited, lookup = 0, [beginWord], set([beginWord]), set(wordList)

        while cur:
            next_queue = []

            for word in cur:
                if word == endWord:
                    return distance + 1
                for i in xrange(len(word)):
                    for j in 'abcdefghijklmnopqrstuvwxyz':
                        candidate = word[:i] + j + word[i + 1:]
                        if candidate not in visited and candidate in lookup:
                            next_queue.append(candidate)
                            visited.add(candidate)
            distance += 1
            cur = next_queue

        return 0


if __name__ == "__main__":
    print Solution().ladderLength("hit", "cog", set(["hot", "dot", "dog", "lot", "log"]))
    print Solution().ladderLength("hit", "cog", set(["hot", "dot", "dog", "lot", "log", "cog"]))