University has gotten too expensive, and you don't really need it to get past these interviews (especially since you have a Master's degree already), you just need a data structures and algorithms book / lecture /class.

I'm someone who graduated with a CS degree over a decade ago and I still have to brush up on data structures and algorithms before I do interviews or else I'll struggle with them also. So don't beat yourself up too much about it.

I know another guy struggling with these things lately, and there's a book that seemed pretty promising, although I haven't read it myself yet to verify. It's called "A Common-Sense Guide to Data Structures and Algorithms, Second Edition" by Jay Wengrow.

MIT OpenCourseWare also has a lecture series for 'Introduction to Algorithms' that I play in the background while doing dishes sometimes to brush up on. They've recorded it a few separate years, so just search data structures and algorithms mit in Youtube and try the different years/teachers until you find one you like. Here's what looks like the most recent one, though, from Spring 2020: https://youtu.be/ZA-tUyM_y7s

You can also sign up for a free self-guided course online, which is what my friend ended up doing.

Also if you're wanting to get a handle on time complexity specifically, you'll want to watch some videos on something called 'Big O Notation'. Like they were probably wanting you to say something like "Well I can solve it in this way, and it's O(n), but I can make it faster by doing this and it'll be much faster at O(n log n)". Here's a video that demonstrates the idea behind it: https://www.youtube.com/watch?v=itn09C2ZB9Y