The C++ solution is flawed. srand() should only be called once for the entire duration of a program. In the C++ solution, the function partition() calls srand(), but partition() is recursively called. This means that srand() is called multiple times, and it resets the PRNG every time it is called. If it is called multiple times in the same second, which is extremely likely, you get the exact same value every time, making it no better than the original QuickSelect algorithm. And that's before getting into the whole "rand() considered harmful" subject.