Shared ancestor of two nodes in a binary tree

Question

A binary tree arrives in level-order form along with two node values. Identify the lowest node whose subtree contains both values.

```python
def solve(raw: str) -> str: ...
```

## Requirements

- Line one is the level-order values separated by commas; `null` marks a missing child and the line may be wrapped in `[...]`.
- Line two holds the two target values separated by a comma.
- Node values are unique, and both targets appear in the tree.
- A node counts as its own descendant, so a parent and child query answers the parent.
- Return the value of the lowest shared ancestor as a decimal string, not its position in the serialization.
- When one candidate ancestor lies below another, report the deeper one.
- `run_case` passes `case["stdin"]` to `solve`.
- Do not append a trailing newline to the returned string; return the value exactly once.