## Description

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:

1. Only one letter can be changed at a time.
2. Each transformed word must exist in the word list.

Note:

• Return 0 if there is no such transformation sequence.
• All words have the same length.
• All words contain only lowercase alphabetic characters.
• You may assume no duplicates in the word list.
• You may assume beginWord and endWord are non-empty and are not the same.

Example 1:

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

Output: 5

Explanation: As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
return its length 5.
```

Example 2:

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

Output: 0

Explanation: The endWord "cog" is not in wordList, therefore no possible transformation.```

bfs

## Python Solution

``````class Solution:
def ladderLength(self, beginWord: str, endWord: str, wordList: List[str]) -> int:
wordset = set(wordList)

queue = deque()
queue.append((beginWord, 1))

word_length = len(beginWord)
while queue:
word, step = queue.popleft()
if word == endWord:
return step
else:
for i in range(0, word_length):
for c in "abcdefghijklmnopqrstuvwxyz":
new_word = word[:i] + c + word[i+1:]
if new_word in wordset:
wordset.remove(new_word)
queue.append((new_word, step + 1))

return 0
``````
• Time Complexity: ~N
• Space Complexity: ~1