ServiceNow visited our campus for recruitment. There was a written test initially, which covered basic computer science questions, including OS, DBMS, and some programming language-related questions, and a few on aptitude. Out of this, 25 students were shortlisted for interviews. I had two technical round interviews and an HR discussion. The number of rounds varied for people, with a maximum of 4 technical rounds.

Given a binary tree, I was asked to print the nodes in all the vertical levels.

                                  1
                               /     \
                            2         3
                          /   \     /   \
                     4        5 6     7

You were supposed to print 4 2 1 5 6 3 7

He was expecting a O(n) solution.