Method Signatures

Create the Social Networking Site

SocialNetworkingSite(List<String> userIds)

• Creates the site with the given unique user IDs.
• Each user initially has no posts, friends, or pending friend requests.

Add a Post

boolean addPost(String userId, String postId, String content)

• Adds a post for the specified user.
• Returns true when the post is added.
• Returns false if the user does not exist, the post ID already exists, or the content is empty.

Like a Post

boolean likePost(String userId, String postId)

• Adds a like from the user to the specified post.
• Returns true when the like is added.
• Returns false if the user or post does not exist, or the user has already liked the post.

Add a Comment

boolean addComment(String userId, String postId, String commentId, String comment)

• Adds a comment to the specified post.
• Returns true when the comment is added.
• Returns false if the user or post does not exist, the comment ID already exists, or the comment is empty.

Send a Friend Request

String sendFriendRequest(String fromUserId, String toUserId)

• Sends a friend request from one user to another.
• Returns "REQUEST_SENT" when the request is created.
• Returns "INVALID_REQUEST" if either user does not exist, both IDs identify the same user, the users are already friends, or a request between them is already pending.

Accept or Reject a Friend Request

String respondToFriendRequest(String fromUserId, String toUserId, boolean accept)

• Processes the pending request sent by fromUserId to toUserId.
• Returns "REQUEST_ACCEPTED" when accept is true.
• Returns "REQUEST_REJECTED" when accept is false.
• Returns "REQUEST_NOT_FOUND" when the matching pending request does not exist.
• An accepted request creates a friendship in both directions.

Get People You May Know

List<String> getPeopleYouMayKnow(String userId, int limit)

• Returns at most limit recommended user IDs.
• Only friends of the user's friends are considered.
• The user, existing friends, and users with a pending friend request in either direction are excluded.
• Recommendations are ordered by the number of mutual friends in descending order.
• Recommendations having the same number of mutual friends are ordered by user ID in lexicographically ascending order.
• Each eligible user appears at most once in the recommendation list.
• Returns an empty list if the user does not exist or no eligible recommendation is available.

General Rules

• User IDs, post IDs, and comment IDs are case-sensitive.
• All IDs and text values contain at least one visible character.
• A user can like a particular post at most once.
• Each post ID and comment ID must be globally unique.
• A friendship is always mutual.
• Rejected friend requests may be sent again later.
• All methods must follow the specified return values without throwing exceptions.

Note:

During the interview round follow-up questions were asked about handling concurrent friend requests. Although you do not have to implement them now.

• How would the system handle a situation in which a single user receives a very large number of friend requests?
• How would concurrency be handled across different operations, and how would dirty reads and race conditions be prevented?
• How would the People You May Know feature be designed, and what recommendation algorithm would it use?
• Where should friendship and connection data be stored, and which type of database should be used for each kind of data?

Constraints

• 1 ≤ userIds.size() ≤ 10,000
• 1 ≤ userId.length() ≤ 50
• 1 ≤ postId.length() ≤ 50
• 1 ≤ commentId.length() ≤ 50
• 1 ≤ content.length() ≤ 5,000
• 1 ≤ comment.length() ≤ 1,000
• 1 ≤ limit ≤ 100
• All values in userIds are unique.

Examples

Example 1: Posts, Likes, and Comments

SocialNetworkingSite(userIds = ["anaya", "kabir", "meera"])
addPost(userId = "anaya", postId = "post-17", content = "My first mountain walk")
Output: true

likePost(userId = "kabir", postId = "post-17")
Output: true

likePost(userId = "kabir", postId = "post-17")
Output: false

addComment(userId = "meera", postId = "post-17", commentId = "comment-8", comment = "Beautiful view!")
Output: true

Example 2: Friend Requests

SocialNetworkingSite(userIds = ["dev", "isha", "neel"])
sendFriendRequest(fromUserId = "dev", toUserId = "isha")
Output: "REQUEST_SENT"

respondToFriendRequest(fromUserId = "dev", toUserId = "isha", accept = true)
Output: "REQUEST_ACCEPTED"

sendFriendRequest(fromUserId = "dev", toUserId = "isha")
Output: "INVALID_REQUEST"

Example 3: People You May Know

SocialNetworkingSite(userIds = ["arun", "bina", "chetan", "diya", "farah"])
sendFriendRequest(fromUserId = "arun", toUserId = "bina")
Output: "REQUEST_SENT"

respondToFriendRequest(fromUserId = "arun", toUserId = "bina", accept = true)
Output: "REQUEST_ACCEPTED"

sendFriendRequest(fromUserId = "bina", toUserId = "chetan")
Output: "REQUEST_SENT"

respondToFriendRequest(fromUserId = "bina", toUserId = "chetan", accept = true)
Output: "REQUEST_ACCEPTED"

sendFriendRequest(fromUserId = "bina", toUserId = "diya")
Output: "REQUEST_SENT"

respondToFriendRequest(fromUserId = "bina", toUserId = "diya", accept = true)
Output: "REQUEST_ACCEPTED"

getPeopleYouMayKnow(userId = "arun", limit = 5)
Output: ["chetan", "diya"]