My first instinct was two stacks, which is the right call, but I fumbled the part where a new operation wipes the redo stack.
Use two stacks (undo and redo) to track cart states, where each operation pushes the previous state onto the undo stack and clears the redo stack. For undo, pop from undo, push current state to redo, and restore the popped state; for redo, do the reverse. Handle empty stacks by returning the current state without error.
Pro tip: Mention that storing deltas or commands instead of full snapshots can be more memory-efficient, but full snapshots simplify correctness and are often acceptable for a shopping cart. Also, clarify that no-ops should not modify the stacks or state.
Confirm what operations are undoable (add, remove, update quantity) and whether undo/redo should be per-user or global. Ask if no-ops should be silently ignored or logged.
Select two stacks (undo and redo) to store cart states or operations. Discuss trade-offs between storing full snapshots vs. deltas/commands.
Implement performOperation: push current state to undo, clear redo, apply operation. Implement undo: if undo not empty, pop, push current to redo, restore. Implement redo: if redo not empty, pop, push current to undo, restore.
Ensure undo/redo on empty stacks are no-ops (return current state). Also handle redo clearing on new operation and potential memory growth.
State time complexity O(1) for undo/redo/perform, space O(n) for n operations. Discuss optimizations like limiting history size or using command pattern.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.