You have two coding tasks. ### Task A: Compute Contamination Spread Time Given an `m x n` grid: - `0` means an empty cell. - `1` means a fresh item. - `2` means a contaminated item. Every minute, each contaminated item contaminates its fresh 4-directional neighbors: up, down, left, and right. Return the minimum number of minutes needed until no fresh items remain. If some fresh item can never be contaminated, return `-1`. Handle edge cases such as an empty grid, no fresh items, isolated fresh items, and multiple initial contaminated items. ### Task B: Implement an In-Memory Transactional Key-Value Store Implement an in-memory key-value store supporting basic operations and transactions. Required API: - `get(key)`: Return the current value for `key`, or `null`/`None` if the key does not exist. - `set(key, value)`: Set `key` to `value` in the current transaction if one exists; otherwise set it in the main store. - `delete(key)`: Delete `key` in the current transaction if one exists; otherwise delete it from the main store. - `begin()`: Start a new transaction. Transactions may be nested. - `rollback()`: Revert all changes made in the most recent active transaction. Return whether the rollback succeeded. - `commit()`: Commit the most recent active transaction. If it is nested, merge its changes into the parent transaction; otherwise apply them to the main store. Return whether the commit succeeded. Important edge cases: - `rollback()` or `commit()` with no active transaction. - Reading a key changed in the current transaction. - Reading a key changed in an outer transaction. - Deleting a key inside a transaction and then rolling back. - Setting a key after deleting it in the same transaction. - Nested transactions with independent rollback and commit behavior.