Is it recommendable to put participants in different conditions across trials?
It depends on what you care to investigate.

Do you care what happens when one person gets two hard questions as compared to someone who gets two easy ones? If so, record it for analysis. If not, control for it by ensuring that each participant receives the same number of easy and difficult problems.

Do you care if there are ordering effects? If so, record that data for analysis. If not, then control for it by ensuring that each participant gets a set of questions classified in the same order. E.g., standardize on EASY-HARD-EASY, and randomly assign problems of each classification.