Lowest Common Ancestor of a Binary Tree III

Question

## Problem

Each node has a `parent` pointer. Given two nodes `p` and `q`, return their lowest common ancestor.

## Example
```
Like finding the intersection of two linked lists going up to the root.
```

## Constraints
- Nodes are guaranteed to exist in the tree

## Approach
Two-pointer technique walking up via parent pointers and switching to the other node's root — O(1) space.