The hardest part here is not the actual generation of code, but to analyze the problem first and to determine what data structures/algorithms are best suited to tackle it. Also, there are a lot of real world examples used in these problems, so the AI would need a general understanding of the modern-day world, which again, is hard.

Lastly, even if the AI figures out the implementation, it has to take care of all the edge cases too, so that the test cases pass. But I suppose this is a trivial problem compared to the other two I mentioned above...