题目描述:
class Solution: def findLadders(self, beginWord: str, endWord: str, wordList: list) -> list: wordList = set(wordList) # 转换为hash实现O(1)的in判断 if endWord not in wordList: return [] # 分别为答案、用于剪枝的已访问哈希,前向分支和后向分支,当前的前向分支以及后向分支中的路径和的长度 # 前向路径分支与后向路径分支的字典结构为{结束词:到达该结束词的路径列表} res, visited, forward, backward, _len = [], set(), {beginWord: [[beginWord]]}, {endWord: [[endWord]]}, 2 while forward: if len(forward) > len(backward): # 始终从路径分支较少的一端做BFS forward, backward = backward, forward tmp = {} # 存储新的前向分支 while forward: word, paths = forward.popitem() # 取出路径结束词以及到达它的所有路径 visited.add(word) # 记录已访问 for i in range(len(word)): for a in 'abcdefghijklmnopqrstuvwxyz': new = word[:i]+a+word[i+1:] # 对结束词尝试每一位的置换 if new in backward: # 如果在后向分支列表里发现置换后的词,则路径会和 if paths[0][0] == beginWord: # 前向分支是从beginWord开始的,添加路径会和的笛卡尔积 res.extend(fPath + bPath[::-1] for fPath in paths for bPath in backward[new]) else: # 后向分支是从endWord开始的,添加路径会和的笛卡尔积 res.extend(bPath + fPath[::-1] for fPath in paths for bPath in backward[new]) if new in wordList and new not in visited: # 仅当wordList存在该词且该词还未碰见过才进行BFS tmp[new] = tmp.get(new, []) + [path + [new] for path in paths] _len += 1 if res and _len > len(res[0]): # res已有答案,且下一次BFS的会和路径长度已超过当前长度,不是最短 break forward = tmp # 更新前向分支 return res