Recently, I was asked this question at an interview, and all I could muster was "I don't remember the most difficult problem - I have worked on lot of tough problems, but it all seemed easy after solving the problem."

Yes, thinking back, I can think of few times I hit roadblocks and come through it step by step. But, in hindsight, it all feels simple - just was tough at the moment. I really feel these as small problems I overcame, not even worth mentioning. Also, I do not even remember the small or big roadblocks hit while working on the problem. Only few of the details of the solution are readily accessible in my mind.

What is the real difficult problem you faced, and how you solved it.. Hope this will help other interviewees like me.

PS: Been thinking about this since reading Elon Musk comment about this as his favourite question: "People [who] really solved the problem, they know exactly how they solved it," Musk explained. "They know the little details." [0]

[0] https://www.cnbc.com/2018/09/20/tesla-elon-musk-ask-all-job-applicants-this-interview-question.html