Is there an $O(n^2)$ or $O(n^3)$ time algorithm to check if a number is a sum of $k$ elements of sorted array? Let $A$ be a sorted array of $n$ positive integers (sorted in non-decreasing order, that is there can be equal consecutive elements). Can we check whether some positive integer $x$ is a sum of $k$ elements of $A$ in $O(n^2)$ or $O(n^3)$ time complexity? If yes what would be the pseudocode?


This seems to be a knapsack problem to me and according to Wikipedia it's an NP-complete problem. So even if the array was unsorted in the first place and we wanted to sort it it would take $O(n\log n)$ time which doesn't really help if the problem is NP-complete anyway. Yet I wonder if some optimization may be made to achieve better time.

Please treat $x$ and $k$ as constants, for running time analysis.
It depends on the numbers in the array. For example if all array elements are positive integers, and the requested sum is S, then O(nS) is easy.