Implement Grid Spread and Transactional Store

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.