public int pop() {
            helper.clear();

            while (!stack.isEmpty())
                helper.push(stack.pop());
            int x = helper.pop();

            if (!helper.isEmpty())
                peek = helper.peek();
            else peek = null;

            while (!helper.isEmpty())
                stack.push(helper.pop());
            return x;
        }
Removes the element from in front of queue and returns that element.
src/problems/easy/ImplementQueueusingStacks.java