https://www.codechef.com/FEB20B/problems/CASH (nice one)
https://en.wikipedia.org/wiki/Steinhaus%E2%80%93Johnson%E2%80%93Trotter_algorithm (permutaion genrator)
https://www.hackerrank.com/challenges/coin-change/problem
Important Que : Kprimes codechef dp : https://www.codechef.com/LRNDSA07/submit/KPRIME