There's a famous question posted on this site which asks about finding the $k$th largest element. Many answers are written there which optimized it and found algorithms with expectation of $O(n)$.
The thing I don't understand is.... Those algorithms wouldn't work if $k$ is dependent on $N$. Therefore $k$ must be a constant.
But if it's a constant why isn't it easiest to simply loop like bubble sort for $k$ times? So you pushed the $k$ largest elements to the end of the array. No? Complexity would be $O(nk)$ but $k$ is a const so it would be $O(n)$.
Why do we need crazy algorithms like median of medians and using quicksort if this alone works?