## Permutation of a list with equals objects - Java

Question
``````List<String> original = Lists.newArrayList("A", "B", "C");
Collection<List<String>> permutations= Collections2.orderedPermutations(original)
``````

gives

``````[A, B, C]
[A, C, B]
[B, A, C]
[B, C, A]
[C, A, B]
[C, B, A]
``````

Great, now I have all possible permutations. But let's say A and C are equal, their order does not matter when they are next to each other.

Wanted result:

``````[A, C, B] or [C, A, B]
[B, A, C] or [B, C, A]
[A, B, C]
[C, B, A]
``````

So we tried using a Comparator

``````    Collection<List<String>> permutations= Collections2.orderedPermutations(original, new Comparator<String>() {
@Override
public int compare(final String o1, final String o2) {
if ((o1.equals("A") && o2.equals("C")) || (o2.equals("A") && o1.equals("C"))) {
return 0;
}
return o1.compareTo(o2);
}
});
``````

But this gave us not the result we expected

``````[A, B, C]
[A, C, B]
``````

Any suggestions how to get the wanted result? Although we used guava, any lib or custom solution is ok.

Show source

## Answers to Permutation of a list with equals objects - Java ( 3 )

1. I would expect this to work. I have not tested.

``````    public int compare(String o1, String o2) {
if (o1.equals("C")) o1 = "A";
if (o2.equals("C")) o2 = "A";
return o1.compareTo(o2);
}
``````

2. If you want to remove permutations that are logically equivalent, then I think you should start by writing out the actual 3-permutations as they would really appear if `A` and `C` were the same thing:

``````[A, B, A]
[A, A, B]
[B, A, A]
[B, A, A]
[A, A, B]
[A, B, A]
``````

With duplicates removed, you would be left with this:

``````[A, B, A]
[A, A, B]
[B, A, A]
``````

By using a `Set<List<String>>` you can keep the duplicates out, i.e.

``````List<String> original = Lists.newArrayList("A", "B", "C");
Collection<List<String>> permutations = Collections2.orderedPermutations(original);

Set<List<String>> permsNoDupes = new HashSet<>(permutations);
``````
3. My suggestion is in pair of ("A", "C") & ("C", "A") always put "A" first and transform it to ("A", "C")

``````    List<String> original = Lists.newArrayList("A", "B", "C");
Set<List<String>> permutations = Collections2.orderedPermutations(original).
stream().
map(list -> {
int indexOfc = list.get(0) == "C" ? 0 : (list.get(1) == "C" ? 1 : -1);
if (indexOfc != -1 && list.get(indexOfc + 1) == "A") {
// modifications not allowed on original list
list = new ArrayList<>(list);
list.set(indexOfc, "A");
list.set(indexOfc + 1, "C");
}
return list;
}).collect(Collectors.toSet());
``````

Results are

``````[[A, B, C], [C, B, A], [B, A, C], [A, C, B]]
``````