## Problem:

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 from beginWord to endWord, or 0 if no such sequence exists.

## Input and output examples

Example 1:

Input: beginWord = “hit”, endWord = “cog”, wordList = [“hot”,”dot”,”dog”,”lot”,”log”,”cog”]

Output: 5

Explanation: One shortest transformation sequence is “hit” -> “hot” -> “dot” -> “dog” -> cog”, which is 5 words long.

Example 2:

Input: beginWord = “hit”, endWord = “cog”, wordList = [“hot”,”dot”,”dog”,”lot”,”log”]

Output: 0

Explanation: The endWord “cog” is not in wordList, therefore there is no valid transformation sequence.

## 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.

## Solution

import collections |

### Test the the solution:

beginWord = "hit" |

More interview questions can be found here:

Algorithm