I had an online test and was asked for a phone interview two days later.

The phone interview was actually quite easy. The task was to find the first duplicated word in a string, like in "sky blue dark blue", where you simply return "blue". I used a hash set, and it was solved! I wrote the code in one shot, and then the interviewer asked about some basic problems of hash tables, time complexity, etc., which I was able to answer very quickly.

Next, they asked how I would solve the problem if the string were so big that it was essentially a book.

After a little communication with the interviewer, I was able to provide the solution as well.

However, I received a rejection email three days later, and I have no idea why.

Actually, all the questions he gave are reasonable and basic problems.