The getRandom part is what makes this tricky.
Combine a dynamic array for O(1) random access with a hash map that stores each element's index in the array. For insert, append to the array and add to the map; for remove, swap the target with the last element, update the map, and pop from the array; getRandom simply picks a random index from the array.
Pro tip: Emphasize that the hash map must store indices, not just presence, and that the swap-with-last trick is what enables O(1) removal. Mention edge cases like removing the last element or when duplicates are allowed (if so, map to a set of indices).
Ask whether duplicates are allowed, whether the data structure needs to handle null values, and if the O(1) is average or worst-case. This shows attention to detail and avoids incorrect assumptions.
State that you'll use a dynamic array (e.g., ArrayList in Java, list in Python) for O(1) random access and a hash map for O(1) lookups. Explain that the map will store the index of each element in the array.
Append the new element to the end of the array and record its index in the hash map. If duplicates are allowed, the map should store a set of indices.
Look up the index of the element to remove. Swap it with the last element in the array, update the moved element's index in the map, then remove the last element from the array and delete the target from the map.
Generate a random index between 0 and array size-1 and return the element at that index. Confirm that all operations are O(1) on average due to hash map lookups and array operations.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.