public static class QueueUsingTwoStacks<T> {
        private Stack<T> stack1 = new Stack<>();
        private Stack<T> stack2 = new Stack<>();
        
        public void enqueue(T a) {
            stack1.push(a);
        }
        
        public T dequeue() {
            fillStack2IfEmpty();
            return stack2.pop();
        }
        
        public T peek() {
            fillStack2IfEmpty();
            return stack2.peek();
        }
        
        private void fillStack2IfEmpty() {
            if (stack2.isEmpty()) {
                while (!stack1.isEmpty()) {
                    stack2.push(stack1.pop());
                }
            }
        }
    }