Word Ladder

Hard~30 min

A transformation sequence from word beginWord to word endWord using a dictionary wordList is a sequence of words beginWord -> s1 -> s2 -> ... -> sk such that:

  • Every adjacent pair of words differs by a single letter
  • Every si for 1 <= i <= k is in wordList. Note that beginWord does not need to be in wordList.
  • sk == endWord

Given two words, beginWord and endWord, and a dictionary wordList, return the number of words in the shortest transformation sequence, or 0 if no such sequence exists.

Examples

Example 1
Input: beginWord = "hit", endWord = "cog", wordList = ["hot", "dot", "dog", "lot", "log", "cog"]
Output: 5
Explanation: One shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog", which has 5 words.
Example 2
Input: beginWord = "hit", endWord = "cog", wordList = ["hot", "dot", "dog", "lot", "log"]
Output: 0
Explanation: The endWord "cog" is not in wordList, so no transformation is possible.

Constraints

  • 1 <= beginWord.length <= 10
  • endWord.length == beginWord.length
  • 1 <= wordList.length <= 5000
  • wordList[i].length == beginWord.length
  • beginWord, endWord, and wordList[i] consist of lowercase English letters
  • beginWord != endWord
  • All the words in wordList are unique
  • Expected time complexity: O(n × m × 26)
Code
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Output

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