So my first thought was a mashup between the two. But that's not quite the mashup that happened. Let me explain.
Well before "Problem", Natalia Kills was trying to be a different kind of artist - less sexpot, more goth. Check out the video - yes, that is Natalia getting tortured. But more to the point, it's very dark and sinister while still remaining fun and danceable. Which is kind of strange. But awesome.
Now, the other "Problem" has some fantastic horns, which makes it ripe for a mashup. Natalia's "Problem" is far more driving guitar and.. well, it's just hot on its own. "Zombie", on the other hand, is more sparse.....
The result? A better song.