IBM Associate Data Scientist interview questions

Applied and next day received a hacker rank. SQL easy, BUT LEETCODE MEDIUM Required depth first search of an acyclic directed graph, and minimize the required changes so that a condition could be met. SQL took me like 8 min but the LEETCODE one I couldn't get right. Code complied and all but didn't pass the test cases and timed out for some. Hadn't seen or practiced a tree traversal question in years much less one that combined trees with minimization problems