I applied for the software engineer position by referral and received an email to set up the first phone call with a recruiter. She then scheduled the first round of technical interviews.

The interviewer spoke Mandarin to me. Initially, he asked questions based on my resume. Subsequently, he inquired about why a hashmap can perform operations in O(1) time.

He then moved on to a coding portion, which included two LeetCode medium problems. I solved both, but for the first one, I used an O(n log n) algorithm instead of the optimal O(n) time complexity.

Two days later, I received a rejection email.

1. Why can a hash map perform O(1)?

2. Two LeetCode Medium problems.