Java语言用两个栈设计一个队列

题目：编写一个类，用两个栈实现队列，支持队列的基本操作（add,poll,peek）

解题：

public class TwoStacksImplementQueue {

        public static class myQueue {

            Stack<Integer> stack1;
            Stack<Integer> stack2;

            public myQueue() {
                stack1 = new Stack<Integer>();
                stack2 = new Stack<Integer>();
            }

            /**
             * add只负责往stack1里面添加数据
             *
             * @param newNum
             */
            public void add(Integer newNum) {
                stack1.push(newNum);
            }

            /**
             * 这里要注意两点：
             * 1.stack1要一次性压入stack2
             * 2.stack2不为空，stack1绝不能向stack2压入数据
             *
             * @return
             */
            public Integer poll() {
                if (stack1.isEmpty() && stack2.isEmpty()) {
                    throw new RuntimeException("Queue is Empty");
                } else if (stack2.isEmpty()) {
                    while (!stack1.isEmpty()) {
                        stack2.push(stack1.pop());
                    }
                }
                return stack2.pop();
            }

            public Integer peek() {
                if (stack1.isEmpty() && stack2.isEmpty()) {
                    throw new RuntimeException("Queue is Empty");
                } else if (stack2.isEmpty()) {
                    while (!stack1.isEmpty()) {
                        stack2.push(stack1.pop());
                    }
                }
                return stack2.peek();
            }
        }

        public static void main(String[] args) {
            myQueue mQueue = new myQueue();
            mQueue.add(1);
            mQueue.add(2);
            mQueue.add(3);
            System.out.println(mQueue.peek());
            System.out.println(mQueue.poll());
            System.out.println(mQueue.peek());
            System.out.println(mQueue.poll());
            System.out.println(mQueue.peek());
            System.out.println(mQueue.poll());
        }
    }