Coursera Algorithms week2 栈和队列 练习测验: Queue with two stacks

题目原文：

Implement a queue with two stacks so that each queue operations takes a constant amortized number of stack operations.

题目要求用栈实现队列的所有操作。

package week2;

import java.util.Stack;

/**
 * Queue with two stacks. Implement a queue with two stacks so that each queue 
 * operations takes a constant amortized number of stack operations
 * @author yangjingjing
 *
 */
public class QueueWith2Stacks<E>{
    Stack<E> inStack = new Stack<E>();
    Stack<E> outStack = new Stack<E>();
    public boolean isEmpty(){
        if(inStack.isEmpty() && outStack.isEmpty())
            return true;
        else return false;
    }
    public int size(){
        return (inStack.size() + outStack.size());
    }
    public void enqueue(E item){
        inStack.push(item);
    }
    public E dequeue(){
        if(outStack.isEmpty()){
            if(inStack.isEmpty()) return null;
            else{
                while(!inStack.isEmpty()) {
                    outStack.push(inStack.pop());
                }
                return outStack.pop();
            }
        }else{
            return outStack.pop();
        }
    }
    public static void main(String[] args) {
        QueueWith2Stacks<Object> queue = new QueueWith2Stacks<Object>();
        queue.enqueue("a");
        queue.enqueue("b");
        queue.enqueue("c");
        System.out.println(queue.dequeue());
        queue.enqueue(1);
        queue.enqueue(2);
        Object o = null;
        while((o = queue.dequeue()) != null) {
            System.out.println(o);
        }
    }
}