Add stub and adapt interface

Author: jcazevedo

src/main/java/com/packt/datastructuresandalg/lesson4/activity/coinchange/CoinChange.java

@@ -0,0 +1,5 @@
+package com.packt.datastructuresandalg.lesson4.activity.coinchange;
+
+public class CoinChange {
+    public Integer ways(int N, int[] coins) { return null; }
+}

src/main/java/com/packt/datastructuresandalg/lesson4/activity/coinchange/solution/CoinChange.java

@@ -1,7 +1,7 @@
 package com.packt.datastructuresandalg.lesson4.activity.coinchange.solution;
 
 public class CoinChange {
-    public int ways(int N, int[] coins) {
+    public Integer ways(int N, int[] coins) {
         int[][] dp = new int[N + 1][coins.length + 1];
         for (int j = 0; j <= coins.length; j++)
             dp[0][j] = 1;