【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）
【代码实现】
import java.util.Stack; class TwoStacksQueue{ public Stack<Integer> stackPush; public Stack<Integer> stackPop; public TwoStacksQueue(){ stackPush = new Stack<Integer>(); stackPop = new Stack<Integer>(); } public void add(int pushInt) { stackPush.push(pushInt); } public int poll(){ if(stackPop.empty() && stackPush.empty()){ throw new RuntimeException("Queue is empty."); }else if(stackPop.empty()){ while(!stackPush.empty()){ stackPop.push(stackPush.pop()); } } return stackPop.pop(); } public int peek(){ if(stackPop.empty() && stackPush.empty()){ throw new RuntimeException("Queue is empty."); }else if(stackPop.empty()){ while(!stackPush.empty()){ stackPop.push(stackPush.pop()); } } return stackPop.peek(); } }