Check if a Binary Tree Is Height-Balanced

Question

## Problem

Return true if, for every node, the heights of the two subtrees differ by at most one.

## Example
```
Balanced BST -> true; degenerate list-like tree -> false
```

## Constraints
- 0 ≤ nodes ≤ 5000

## Approach
Bottom-up height returning -1 to short-circuit imbalance. O(n).