This issue is about [Diameter of BST]

https://www.geeksforgeeks.org/diameter-of-a-binary-tree/

• Issue Title is in form Algo/DS Name [Language].
• I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• This Algo/DS is worth adding to this repository.
• Assign this issue to me (I want to work on this).
  • 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.
• I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

Kindly assign me this issue under Contribute-a-thon'21.