public String toString() 
    { 
	//simple version:
	//return _heap.toString(); 

	//prettier version:
	String lvlOrdTrav = "heap size " + _heap.size() + "\n";

	if ( _heap.size() == 0 ) return lvlOrdTrav;

	int h = 1; //init height to 1
	for( int i = 0; i < _heap.size(); i++ ) {
	    lvlOrdTrav += i + ":" + _heap.get(i) + " ";
	    if ( i >= Math.pow(2,h) - 2 ) {
		lvlOrdTrav += "\n";
		h++;
	    }
	}
	return lvlOrdTrav;
    }//O(n)