Design a Set of Stacks with `popAt`

To solve this, I would first define a SetOfStacks class containing a list of individual Stack objects, each with a defined capacity. For the push operation, I'd check if the last stack in the list has reached its limit. If it has, I create a new Stack instance and append it to the list before pushing the element. This ensures we never exceed the capacity constraint. The popAt operation is more complex. Given an index, I access the corresponding sub-stack directly. If the index is out of bounds, I return null. Otherwise, I perform a standard pop on that specific stack. A crucial discussion point here is whether to shift elements down. In many real-world scenarios, especially at Adobe where performance matters, we might avoid shifting to keep O(1) access, accepting that lower stacks might become sparse. However, if the requirement is to keep stacks as full as possible, I would implement a recursive shift-down mechanism that moves the bottom element of the next stack up to fill the gap. I'd also consider edge cases, such as popping from an empty set or accessing a negative index. Finally, I'd analyze the space complexity, which is linear relative to the total number of elements, and time complexity, highlighting that while push is constant time, popAt could be linear if shifting is mandatory.