I have my graph implemented with linked lists, for both vertices and edges and that is becoming an issue for the Dijkstra algorithm. As I said on a previous question, I’m converting this code that uses an adjacency matrix to work with my graph implementation.
The problem is that when I find the minimum value I get an array index. This index would have match the vertex index if the graph vertexes were stored in an array instead. And the access to the vertex would be constant.
I don’t have time to change my graph implementation, but I do have an hash table, indexed by a unique number (but one that does not start at 0, it’s like 100090000) which is the problem I’m having. Whenever I need, I use the modulo operator to get a number between 0 and the total number of vertices.
This works fine for when I need an array index from the number, but when I need the number from the array index (to access the calculated minimum distance vertex in constant time), not so much.
I tried to search for how to inverse the modulo operation, like, 100090000 mod 18000 = 10000 and, 10000 invmod 18000 = 100090000 but couldn’t find a way to do it.
My next alternative is to build some sort of reference array where, in the example above, arr[10000] = 100090000. That would fix the problem, but would require to loop the whole graph one more time.
Do I have any better/easier solution with my current graph implementation?
In your array, instead of just storing the count (or whatever you’re storing there), store a structure which contains the count as well as the vertex index number.