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    <p>Linked and doubly-linked lists are a specific variety of <strong>directed graphs</strong> which can be optimized into the <code>head/tail</code>, <code>data/next/prev</code> structure you're familiar with. Since you're broadening its capabilities, you lose that specificity, and want to go back to the generic directed graph structure and work from there.</p> <p>A directed graph is most easily described with an adjacency list: <img src="https://i.stack.imgur.com/0pOWE.gif" alt="image of graphs with adjacency lists"></p> <p>You can implement that with as a list of lists, or an array of lists, or a jagged array, or however you like. Now, on the right, I've drawn a doubly-linked list in directed graph form. Since the <code>next</code> pointers are different from <code>prev</code> pointers, your adjacency list needs to keep those separate. So it will actually be a list of dual lists:</p> <pre><code>typedef struct _BPNode { // "Back-Pointing Node" void *data; struct _BPNode *nexts[]; struct _BPNode *prevs[]; } Node; typedef struct _BPGraph { // "Back-Pointing Graph" Node **allNodes; } BPGraph; </code></pre> <p>Or something like that. Disclaimer: I didn't test this in a compiler. And just in case, <a href="http://unixwiz.net/techtips/reading-cdecl.html" rel="nofollow noreferrer">here's a guide on how to read some of the declarations in there</a>.</p> <p>Alternatively, you can create two directed graphs, one running forward, and one running backward. However, that would take more memory than this "back-pointing" graph. It would also run slower (more cpu cache misses), would be less intuitive, and would be more troublesome to free memory for.</p>
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