Two stacks, one for undo history and one for redo, felt like the obvious move.
Start by clarifying requirements: operations are cell value assignments, undo/redo can be applied in bulk across k steps. Then propose a data structure combining a doubly linked list of operations for O(1) undo/redo and a hash map from cell ID to a stack of previous values to efficiently restore cell states. Discuss trade-offs between memory and speed, and how to handle bulk operations by moving a pointer k steps and applying inverses.
Pro tip: Mention that you would use a sentinel node to simplify edge cases in the doubly linked list, and that you would consider lazy deletion or periodic compaction to manage memory growth from the undo history.
Ask about the scale (number of cells, operations), whether undo/redo history is bounded, and if operations are only cell sets or include other types. Confirm that bulk undo/redo means moving k steps at once.
Propose a doubly linked list where each node represents an operation (cell ID, old value, new value) and a hash map from cell ID to a stack of previous values for O(1) access to the last value. Explain how the linked list supports O(1) undo/redo by moving a pointer.
For bulk undo of k steps, traverse the linked list backwards k nodes, applying the inverse of each operation (restoring old values) and updating the cell value stacks. For redo, traverse forwards. Discuss time complexity O(k) and how to optimize if k is large.
Discuss what happens when undo/redo reaches the beginning/end of history, how to handle new operations after undo (truncate redo history), and memory management (e.g., limiting history size, using persistent data structures).
Summarize the design, its time/space complexity, and potential extensions like supporting multiple users or collaborative editing. Mention how this scales for Airtable's use case.
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