图论的常见题目有两类，一类是求两点间最短距离，另一类是拓扑排序，两种写起来都很烦。

求最短路径：

127. Word Ladder

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:

1. Only one letter can be changed at a time
2. Each intermediate word must exist in the word list

For example,

Given:
beginWord = "hit"
endWord = "cog"
wordList = ["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.

从起点开始向外更新，因为每条路径的权值都不是负数，所以先更新的总比后更新的小。

已经被更新过的之后就不用考虑了

[leetcode] 题型整理之图论