Word Ladder (Shortest Transformation)

Question

## Problem

Shortest transformation sequence length from beginWord to endWord changing one letter at a time within a word list.

## Example
```
hit -> cog via [hot,dot,dog,lot,log,cog] -> 5
```

## Constraints
- 1 ≤ wordList ≤ 5000

## Approach
BFS over the word graph; bidirectional BFS to optimize.