算法Sedgewick第四版-第1章基础-022一QueueWithTwoStacks

 /******************************************************************************
  *  Compilation:  javac QueueWithTwoStacks.java
  *  Execution:    java QueueWithTwoStacks < input.txt
  *  Dependencies: StdIn.java StdOut.java
  *  Data files:   http://algs4.cs.princeton.edu/13stacks/tobe.txt
  *
  *  A generic queue, implemented using two stacks.
  *
  *  % java QueueWithTwoStacks < tobe.txt
  *  to be or not to be (2 left on queue)
  *
  ******************************************************************************/

 import java.util.NoSuchElementException;

 public class QueueWithTwoStacks<Item> {
     private Stack<Item> stack1;    // back of queue
     private Stack<Item> stack2;    // front of queue

     // create an empty queue
     public QueueWithTwoStacks() {
         stack1 = new Stack<Item>();
         stack2 = new Stack<Item>();
     }

     // move all items from stack1 to stack2
     private void moveStack1ToStack2() {
         while (!stack1.isEmpty())
             stack2.push(stack1.pop());
     }

     // is the queue empty?
     public boolean isEmpty() {
         return stack1.isEmpty() && stack2.isEmpty();
     }

     // return the number of items in the queue.
     public int size() {
         return stack1.size() + stack2.size();
     }

     // return the item least recently added to the queue.
     public Item peek() {
         if (isEmpty()) throw new NoSuchElementException("Queue underflow");
         if (stack2.isEmpty()) moveStack1ToStack2();
         return stack2.peek();
     }

     // add the item to the queue
     public void enqueue(Item item) {
         stack1.push(item);
     }

     // remove and return the item on the queue least recently added
     public Item dequeue() {
         if (isEmpty()) throw new NoSuchElementException("Queue underflow");
         if (stack2.isEmpty()) moveStack1ToStack2();
         return stack2.pop();
     }

     // a test client
     public static void main(String[] args) {
         QueueWithTwoStacks<String> q = new QueueWithTwoStacks<String>();
         while (!StdIn.isEmpty()) {
             String item = StdIn.readString();
             if (!item.equals("-")) q.enqueue(item);
             else if (!q.isEmpty()) StdOut.print(q.dequeue() + " ");
         }
         StdOut.println("(" + q.size() + " left on queue)");
     }
 }