兩個棧實現一個隊列

package java_study.JianZhiOffer;

import java.util.Stack;

/**
 * Created by ethan on 2015/6/22.
 */
public class No7Queue {
    public Stack<Integer> stackInput;
    public Stack<Integer> stackOutput;
    public No7Queue(){
        stackInput = new Stack<Integer>();
        stackOutput = new Stack<Integer>();
    }
    public void push(int x){
        stackInput.push(x);
    }
    public boolean isEmpty() throws Exception{
        if (stackInput==null || stackOutput==null)
            throw new NullPointerException();
        if (stackInput.isEmpty()&&stackOutput.isEmpty())
            return true;
        else
            return false;
    }
    public int pop() throws Exception{
        if (this.isEmpty()) throw new Exception("this Queue is empty");
        if (stackOutput.isEmpty()){
            while (!stackInput.isEmpty()){
                stackOutput.push(stackInput.pop());
            }
        }
        return stackOutput.pop();
    }
    public int top() throws Exception{
        if (this.isEmpty()) throw new Exception("this Queue is empty");
        if (stackOutput.isEmpty()){
            while (!stackInput.isEmpty()){
                stackOutput.push(stackInput.pop());
            }
        }
        return stackOutput.peek();
    }
}