I'm trying to solve Power sum on codechef https://www.codechef.com/SEPT19A/problems/PSUM, the problem can be solved using Dp and NTT. however when I viewed the submissions I noticed that the solution involves using the power series, can somebody please explain the intuition behind the solution and why do we use the power series? thats one of the submissions to the problem https://www.codechef.com/viewsolution/26607700 .