How to convert Set<Set<String>> to Set<String>?

Assuming that Streams and Collections, Lambdas are allowed to use? I tried using a for loop but it didn't solve my problem.

// Set<Set<String>> to Set<String>
for(Set<String> s : set) {
    result.addAll(s);
    set.add(result);
}

set is a Set<Set<String>> type and result is a type of Set<String>.

Here is the option using Stream API:

Set<String> result = sets.stream()
    .flatMap(Collection::stream)
    .collect(Collectors.toSet());