// Program to demonstrate the usage of stacks by 
//   modeling a browser back button
import java.util.*;

public class Browser {
   public static void main(String[] args) {
      Stack<String> history = new Stack<String>();
      
      visit("www.google.com", history);
      visit("cs.washington.edu/143x", history);
      System.out.println(history);
      visit("www.facebook.com", history);
      System.out.println(history);
      System.out.println();
      
      goBack(history);
      System.out.println(history);
      visit("www.fivethirtyeight.com", history);
      System.out.println(history);
      goBack(history);
      goBack(history);
      goBack(history);
      System.out.println(history);
   }
   
   // visit the given page
   // pre: history != null
   public static void visit(String page, Stack<String> history) {
      System.out.println("Visiting " + page);
      history.push(page);
   }
   
   // go back from the current page to the previous one
   // pre: history != null
   public static void goBack(Stack<String> history) {
      String lastPage = history.pop();
      System.out.println("Leaving " + lastPage + "...");
      
      if (history.isEmpty()) {
         System.out.println("  No pages left.");
      } else {
         String newPage = history.pop();
         System.out.println("  Back to " + newPage);
         history.push(newPage);
      }
   }
}