Generate a String From Binary Tree Node Values

Question

## Problem

Given a binary tree, build a string encoding its structure using node values and parentheses for children.

## Example
```
[1,2,3,null,4] -> "1(2()(4))(3)"
```

## Constraints
- Preorder with parentheses; omit empty trailing parens

## Approach
Preorder DFS; careful with when to emit '()' for a missing left child. Reported Fastly question.