文字题解与代码:https://leetcode-cn.com/problems/word-ladder/solution/yan-du-you-xian-bian-li-shuang-xiang-yan-du-you-2/。
扩展问题:「力扣」第 126 题:单词接龙 II,题解:https://leetcode-cn.com/problems/word-ladder-ii/solution/yan-du-you-xian-bian-li-shuang-xiang-yan-du-you--2/