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();
}
}