During the interview, I approached the problem of designing a file system by first outlining the main components and operations. I started by distinguishing the responsibilities of each method: mkdir for creating directories, ls for listing files and directories, and addContentToFile for adding content to existing files. I emphasized that I wanted to handle each element with clarity and ensure a scalable design.

I described a tree data structure to represent the file and directory hierarchy. Each directory would be a node containing a dictionary for its files and subdirectories. This way, mkdir would navigate through the directory tree, creating new nodes as needed. For ls, I would implement a function that traverses the directory and collects the names of files and directory names in a structured manner. I could also leverage depth-first or breadth-first search depending on how I wanted to present the data.

The interviewer seemed engaged with this design choice and prompted me to elaborate on how I would handle edge cases, such as trying to create a directory that already exists. I explained an approach where mkdir would check for the existence of the node before creating it. I also discussed how I would handle concurrent modifications, a common issue in file systems. Here, I suggested using locks or timestamps to manage access safely.

As we moved forward, the interviewer asked follow-up questions around memory efficiency, specifically how I would store large files. I answered by explaining that I could implement chunking, where a file would be divided into smaller parts. This would allow the system to only load parts of a file into memory as needed. My explanation seemed to resonate well with the interviewer, and I could sense their appreciation for how I tackled both functional and non-functional requirements.

Ultimately, we discussed potential performance implications and I suggested that caching frequently accessed directories or files could improve responsiveness. The interviewer encouraged me to think critically about how these optimizations would scale as the file system grew.

Overall, I felt that I had communicated my thought process clearly and demonstrated strong problem-solving skills throughout the interview. The interviewer concluded by thanking me for my detailed responses and mentioned that they would be in touch soon. I left the interview feeling optimistic about my performance and the fit with the Pinterest team.