CC3.3.java

public class SetOfStack{
    
    private ArrayList<Stack<Integer>> lstStack;
    private int capacity;
    
    public SetOfStack(int n) {
        this.capacity = n;
        this.lstStack = new ArrayList<Stack<Integer>>();
    }
    
    public void push(int val) {
        if(lstStack.size() == 0 || lstStack.get(lstStack.size()-1).size() == capacity) {
            Stack<Integer> stack = new Stack<Integer>();
            lstStack.add(stack);
        }
        lstStack.get(lstStack.size()-1).push(val);
    }
    
    public int pop() {
        if(lstStack.size() == 0) {
            throw new NoSuchElementException("stack underflow!");
        }
        int idx = lstStack.size()-1;
        Stack<Integer> stack = lstStack.get(idx);
        int val = stack.pop();
        if(stack.size() == 0)
            lstStack.remove(idx);
        return val;
    }
    
    public int popAt(int idx) {
        if(lstStack.size() == 0)
            throw new NoSuchElementException("stack underflow!");
        if(idx >= lstStack.size())
            throw new NoSuchElementException("stack overflow!");
        Stack<Integer> stack = lstStack.get(idx);
        int val = stack.pop();
        if(stack.size() == 0)
            lstStack.remove(idx);
        return val;
    }
    
}

1. Consider to move next element to current stack after pop() operation.