2.由两个栈组成的队列

题目：用两个栈实现队列，能够支持级别的操作（add  poll  peek）

思路：用一个栈存数据，另一个负责取数据，当取数据的栈空的时候，就把存数据的那个栈的数据全部取过来就行了，当两个栈都为空，则队列为空

import java.util.Stack;public class StackToQueue {public Stack<Integer> inStack = new Stack<Integer>();public Stack<Integer> outStack = new Stack<Integer>();public void add(int e){inStack.push(e);}public int poll(){if(inStack.isEmpty()&&outStack.isEmpty())throw new RuntimeException("stack is empty ");else if(outStack.isEmpty()){while(!inStack.isEmpty()){outStack.push(inStack.pop());}}return outStack.pop();}public int peek(){if(inStack.isEmpty()&&outStack.isEmpty())throw new RuntimeException("stack is empty ");else if(outStack.isEmpty()){while(!inStack.isEmpty()){outStack.push(inStack.pop());}}return outStack.peek();}}