<p>Imagine a (literal) stack of plates. If the stack gets too high, it might topple. Therefore, in real life, we would likely start a new stack when the previous stack exceeds some threshold. Implement a data structure <code>SetOfStacks</code> that mimics this. <code>SetOfStacks</code> should be composed of several stacks and should create a new stack once the previous one exceeds capacity. <code>SetOfStacks.push()</code> and <code>SetOfStacks.pop()</code> should behave identically to a single stack (that is, <code>pop()</code> should return the same values as it would if there were just a single stack). Follow Up: Implement a function <code>popAt(int index)</code> which performs a pop operation on a specific sub-stack.</p>
<p>You should delete the sub-stack when it becomes empty. <code>pop</code>, <code>popAt</code> should return -1 when there's no element to pop.</p>
