import java.util.*;

public class BrowserHistory 
{
	private LinkedList myBack,myNext;
	private String myCurrent;
	
	public BrowserHistory()
	{
		myBack = new LinkedList();
		myNext = new LinkedList();
		myCurrent = null;
	}
	
	public String getCurrent()
	{
		return myCurrent;
	}
	
	public void add(String url)
	{
		if (myCurrent != null){
			myBack.addLast(myCurrent);		
		}
		myCurrent = url;
	}
	public String next()
	{
		return null;
	}
	
	public String back()
	{
		myNext.addLast(myCurrent);
		myCurrent = (String) myBack.getLast();
		myBack.removeLast();
		return myCurrent;
	}
	public boolean hasBack()
	{
		return myBack.size() != 0;
	}
	public boolean hasNext()
	{
		return myNext.size() != 0;
	}
}