Word Ladder — Shortest Transformation Sequence Length

Question

## Problem

Given a `beginWord`, `endWord`, and a `wordList`, return the **length of the shortest transformation sequence** from `beginWord` to `endWord` where each step changes exactly one letter and each intermediate word must be in `wordList`.

Return `0` if no such sequence exists.

## Example
```
beginWord = "hit", endWord = "cog"
wordList = ["hot","dot","dog","lot","log","cog"]
Output: 5  -- hit → hot → dot → dog → cog
```

## Constraints
- 1 ≤ wordList.length ≤ 5000
- All words have the same length

## Follow-up (Word Ladder II)
Return **all** shortest transformation sequences. This is significantly harder — BFS + backtracking.