Educative (WA) Software Developer interview questions

It was the first round, basically a screening round where some senior engineer came, and asked me leetcode medium questions to solve with him. First, they asked me the a fibonacci series question, it was easy but the way he framed it and asked to return output was actually a bit confusing and i messed things up a bit. He wanted me to return the nth fibonacci number. The function would be passed a number n, and we would have to return that specific fibonacci number. I messed up when i ended up going one iteration ahead than what was required. it was on paper. The second question was a bit difficult, he asked me to return the nth largest number from a binary search tree without using recursion or a stack. Because, he wanted me to have O(1) space complexity. There was a bit complicated algorithm behind it which i didn't know at that time, hence, I couldn't do this question. My way of trying to reach the solution was a bit flawed too.