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66 lines
2.4 KiB
Markdown
Executable File
66 lines
2.4 KiB
Markdown
Executable File
# [127. Word Ladder](https://leetcode.com/problems/word-ladder/)
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## 题目
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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:
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1. Only one letter can be changed at a time.
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2. Each transformed word must exist in the word list. Note that *beginWord* is *not* a transformed word.
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**Note:**
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- Return 0 if there is no such transformation sequence.
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- All words have the same length.
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- All words contain only lowercase alphabetic characters.
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- You may assume no duplicates in the word list.
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- You may assume *beginWord* and *endWord* are non-empty and are not the same.
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**Example 1:**
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Input:
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beginWord = "hit",
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endWord = "cog",
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wordList = ["hot","dot","dog","lot","log","cog"]
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Output: 5
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Explanation: As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
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return its length 5.
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**Example 2:**
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Input:
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beginWord = "hit"
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endWord = "cog"
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wordList = ["hot","dot","dog","lot","log"]
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Output: 0
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Explanation: The endWord "cog" is not in wordList, therefore no possible transformation.
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## 题目大意
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给定两个单词（beginWord 和 endWord）和一个字典，找到从 beginWord 到 endWord 的最短转换序列的长度。转换需遵循如下规则：
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1. 每次转换只能改变一个字母。
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2. 转换过程中的中间单词必须是字典中的单词。
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说明:
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- 如果不存在这样的转换序列，返回 0。
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- 所有单词具有相同的长度。
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- 所有单词只由小写字母组成。
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- 字典中不存在重复的单词。
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- 你可以假设 beginWord 和 endWord 是非空的，且二者不相同。
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## 解题思路
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- 这一题要求输出从 `beginWord` 变换到 `endWord` 最短变换次数。可以用 BFS，从 `beginWord` 开始变换，把该单词的每个字母都用 `'a'~'z'` 变换一次，生成的数组到 `wordList` 中查找，这里用 Map 来记录查找。找得到就入队列，找不到就输出 0 。入队以后按照 BFS 的算法依次遍历完，当所有单词都 `len(queue)<=0` 出队以后，整个程序结束。
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- 这一题题目中虽然说了要求找到一条最短的路径，但是实际上最短的路径的寻找方法已经告诉你了：
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1. 每次只变换一个字母
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2. 每次变换都必须在 `wordList` 中
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所以不需要单独考虑何种方式是最短的。
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