costs

//iterate until there are no elements
    int u, v, c;
    while(not pq.empty()){
        pq.max(u,v,c);
        sides[u] = not sides[u];
        sides[v] = not sides[v];

        //uptate cost or delete
        int cost = get_cost(u, v, neighbours, neighbours_position, sides);
        if(cost > 0) pq.assign(u, v, cost);
        else pq.erase(u, v);
    }