import java.util.*;

public class Tokenizer {

    private String pushed = null;

    private StringTokenizer tokens;

    public Tokenizer(String s) {
	tokens = new StringTokenizer(s, "()*/+-", true);
    }

    /**
     * @return null if no more tokens left
     **/
    public String nextToken()
    {
	String token;
	if (pushed != null) {
	    token = pushed;
	    pushed = null;
	}
	else {
	    token = "";
	    while (token.equals("")) {
		try {
		    token = tokens.nextToken().trim();
		}
		catch (NoSuchElementException e) {
		    return null;
		}
	    }
	}
	//System.out.println("'" + token + "'");
	return token;
    }

    public void pushBack(String token) {
	pushed = token;
    }

    public boolean hasMoreTokens() {
	return tokens.hasMoreTokens() || (pushed != null);
    }

}