Disjoint Union Set [Java]

[SauravP97]

This issue is about Disjoint Union Set.

• [x] Issue Title is in form Algo/DS Name [Language].
• [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• [x] This Algo/DS is worth adding to this repository.
• [x] Assign this issue to me (I want to work on this).
  • [x] I ensure that I am not already assigned to 2 or more issues.
• [ ] I will send my PR only after I'm assigned to this issue by any maintainer.
• [x] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

[vidit2512]

@iiitv/project-maintainers I want to work on this issue.

[theyashshahs]

@vidit2512 GFI 👍