The first step was a phone screening with a recruiter, during which we discussed my background and experience. The recruiter also provided information about the company culture and the position I was applying for.

Next, I had two technical phone interviews with software engineers on the team. These interviews were challenging and covered a wide range of topics, such as algorithms, data structures, and programming languages. However, the interviewers were very friendly and supportive, and did a good job of guiding me through the questions.

After successfully completing the phone interviews, I was invited to an onsite interview at the Microsoft campus. This consisted of several rounds of interviews with different members of the team, including technical and behavioral interviews. The technical interviews were quite challenging, but I felt well-prepared and was able to perform well.

Given a binary tree, write a function to check if it's a valid binary search tree.