Question

For example suppose there are 3 nodes A,B,C and A links to B and C, B links to A and C, and C links to B and A. In visual form its like this

C <- A -> B //A links to B & C
A <- B -> C //B links to A & C
B <- C -> A //C links to B & A

Assume the A,B,C are held in an array like so [A,B,C] with index starting at 0. How can I efficiently sort the array [A,B,C] according to the value held by each node.

For example if A holds 4, B holds -2 and C holds -1, then sortGraph([A,B,C]) should return [B,C,A]. Hope its clear. Would it be possible if I can somehow utilize std::sort?

EDIT: Not basic sort algorithm. Let me clarify a bit more. Assume I have a list of Nodes [n0,n1…nm]. Each ni has a left and right neighbor index. For example, n1 left neight is n0 and its right neighbor is n2. I use index to represent the neighbor. If n1 is at index 1, then its left neighbor is at index 0 and its right neighbor is at index 2. If I sort the array, then I need to update the neighbor index as well. I don’t want to really implement my own sorting algorithm, any advice on how to proceed?

Answer 1

If I understand the edited question correctly your graph is a circular linked list: each node points to the previous and next nodes, and the “last” node points to the “first” node as its next node.

There’s nothing particularly special you need to do the sort that you want. Here are the basic steps I’d use.

1. Put all the nodes into an array.
2. Sort the array using any sorting algorithm (e.g. qsort).
3. Loop through the result and reset the prev/next pointers for each node, taking into account the special cases for the first and last node.