// Recursive algo is different from Binomial Identity
                            // Given coins[k] = {2, 3, 4} and change s = 9 
                            // Count the number of way for coin change 
                            // coins[k] is in the each change
                            // coins[k] is not in the each change 
                            // count(coins, k, s) = count(coins, k-1, s) + count(coins, k, s-coins[k]) 
                            public static int count(int[] coin, int s, int k) {
                                if( s < 0)
                                    return 0;
                                else if( s == 0 ) 
                                    return 1;
                                else if( s > 0 && k > 0) {
                                    int left = count(coin, s, k-1);
                                    int right= count(coin, s-coin[k-1], k);
                                    return left + right;
                                }
                                else 
                                    return 0;
                            }