Phillips 66 Software Developer interview questions

1- OA round had 1 coding and 30 mcq question based in cs fundamentals. 2-technical interview- the interviewer asked to introduce myself. then he moved towards oops concept , i was grilled over that for 20 minutes then he moved to some basic javascript questions, after this he asked me to reverse a string , reverse a number. the whole round went for about 30 minutes. 3-HR round - started with introduction, then he moved towards some technical questions like regression testing and some system design questions. also asked some situation based question, it went around 30 minutes.

easy but difficultA straightforward solution using O(mn) space is probably a bad idea. A simple improvement uses O(m + n) space, but still not the best solution. Could you devise a constant space solution?