When you do oMap.erase(it->first);, you're essentially erasing it. Then, you try to increment it, which causes undefined behavior. This can be tested by adding printing it->first every time you run the inner for loop. CodeForces and GeeksForGeeks will give different outputs.

Also, I don't think using a map to store values of m will make your code any faster, since the maximum values of n and k are the same. So using a map adds an unnecessary log n factor.