void dijkstra(string s, NodeMap &nodes)
{
	// check and report or abort
	Node* source = nodes.nodemap[s];
	if(source==0)
    {
                 cout << "Sorry we don't fly from "<< s << endl; return;
    }
	reset_nodes(nodes);
 
	// put the source into pq and loop until empty
	priority_queue<Node *, deque<Node*>, compare>pq;
	pq.push(source);
	while(!pq.empty())
    {
		// process least cost node.
		Node* curr = pq.top(); 
		pq.pop();
		curr->visited = true;
		// process neighbors
		list<Edge*>::iterator edge;
		for(edge = curr->neighbors.begin(); edge != curr->neighbors.end(); edge++)
        {
			Node *ne = (*edge)->dest;
			if(!ne->visited)
            {
				ne->cost += (*edge)->weight + curr->cost;
				ne->visited = true;
				ne->back_pointer = curr;
				cout << "Cheapest price to "<< ne->name << "                  = " << ne->cost << endl;;
				pq.push(ne);
			}		
        }
	}
}