@Override protected void addToFrontier(Node node){
  if (!explored.contains(node.getState())) {
    frontier.add(node);
    updateMetrics(frontier.size());
  }
}

@ Override protected void addToFrontier ( Node node ) { if ( ! explored . contains ( node . getState ( ) ) ) { frontier . add ( node ) ; updateMetrics ( frontier . size ( ) ) ; } }
inserts the node at the tail of the frontier if the corresponding state was not yet explored .
