# [126. Word Ladder II](https://leetcode.com/problems/word-ladder-ii/)


## 题目

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

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

**Note:**

- Return an empty list 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:
    [
      ["hit","hot","dot","dog","cog"],
      ["hit","hot","lot","log","cog"]
    ]

**Example 2:**

    Input:
    beginWord = "hit"
    endWord = "cog"
    wordList = ["hot","dot","dog","lot","log"]
    
    Output: []
    
    Explanation: The endWord "cog" is not in wordList, therefore no possible transformation.

## 题目大意

给定两个单词（beginWord 和 endWord）和一个字典 wordList，找出所有从 beginWord 到 endWord 的最短转换序列。转换需遵循如下规则：

1. 每次转换只能改变一个字母。
2. 转换过程中的中间单词必须是字典中的单词。

说明:

- 如果不存在这样的转换序列，返回一个空列表。
- 所有单词具有相同的长度。
- 所有单词只由小写字母组成。
- 字典中不存在重复的单词。
- 你可以假设 beginWord 和 endWord 是非空的，且二者不相同。



## 解题思路

- 这一题是第 127 题的加强版，除了找到路径的长度，还进一步要求输出所有路径。解题思路同第 127 题一样，也是用 BFS 遍历。
- 当前做法不是最优解，是否可以考虑双端 BFS 优化，或者迪杰斯塔拉算法？