I came across the following question.
Given an array of n elements and an integer k where k < n. Elements {a0...ak} and {ak+1...an} are already sorted. Give an algorithm to sort in O(n) time and O(1) space.
It does not seem to me like it can be done in O(n) time and O(1) space. The problem really seems to be asking how to do the merge step of mergesort but in-place. If it was possible, wouldn't mergesort be implemented that way? I am unable to convince myself though and need some opinion.