Only one letter can be changed at a time Each intermediate word must exist in the dictionary For example, , All words have the same length. All words contain only lowercase alphabetic characters. 分析：这种题，肯定是每次改变单词的一个字母，然后逐渐搜索，很多人一开始就想到用dfs，其实像这种求最短路径、树最小深度问题bfs最适合，可以参考我的这篇博客 bfs（层序遍历）求二叉树的最小深度 。本题bfs要注意的问题： 和当前单词相邻的单词是：对当前单词改变一个字母且在字典中存在的单词 bfs队列中用NULL来标识层与层的间隔，每次碰到层的结尾，遍历深度+1 Only one letter can be changed at a time Each intermediate word must exist in the dictionary For example, All words contain only lowercase alphabetic characters.
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