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);
         }
     }
 }