Word Ladder

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

1. Only one letter can be changed at a time
2. 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.

思路1：bfs，遍历str的每个字母，进行修改，如果是在字典中，且之前并未出现过的字符串（防止出现循环），则将其放入到队列中

思路2：根据思路1，有点图的感觉。。。最短路径？

思路3：dfs，做肯定可以做，但其实没必要，因为并不是一个（现在发现其实dfs是有问题的，因为路径长短的问题）