Question

I’m having a bit of trouble reversing a given map and storing its reversed keys and values into another map. I have a method prototype as follows:

public static Map<String, Set<String>> reverse (Map <String, Set<String>> graph);

So if I have sample keys for the directed graph such that:

{c -> arraySet{f, e}}
{b -> d}
{a -> arraySet{c, b}} 
{d -> g}
{e -> d}
{f -> arraySet{g, d}}

I need to effectively reverse this graph so that instead of b -> d I have d -> b.

I think all this requires is for me is to interchange the values and keys in the original graph and add them to the reverseMap. I suppose I could iterate through each set of values for a given key in the graph and then and store those in a list.

Unfortunately, I’m having trouble implementing this and thinking it through. I’d really appreciate a nudge in the right direction.

Answer 1

You will need to loop over the entries in your map, and then, since the values are stored in a set, you will need to loop over that set. You will need to check your result map for each key and create a new set whenever a key does not yet exist.

public static Map<String, Set<String>> reverse (Map <String, Set<String>> graph) {
    Map<String, Set<String>> result = new HashMap<String, Set<String>>();
    for (Map.Entry<String, Set<String>> graphEntry: graph.entrySet()) {
        for (String graphValue: graphEntry.getValue()) {
            Set<String> set = result.get(graphValue);
            if (set == null) {
                set = new HashSet<String>();
                result.put(graphValue, set);
            }
            set.add(graphEntry.getKey());
        }
    }
    return result;
}