Word Search II (Trie + DFS)

Question

## Problem

Given a board of letters and a word list, return all words that can be formed by adjacent cells.

## Example
```
find all dictionary words present in the grid
```

## Constraints
- Up to 10^4 words

## Approach
Build a trie of words, DFS the board pruning by trie. Snowflake favors graph/trie problems.