栈和队列——由两个栈组成的队列

【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）

【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();    }}