Added sum with recursion to 03_recursion, java folder (#254)

* added sum with recursion to 03_recursion, java folder

* Update .gitignore

* update list not to be modified during recursion

* fixed update

03_recursion/java/04_sum/src/Sum.java

@@ -0,0 +1,19 @@
+import java.util.*;
+
+public class Sum {
+    public static int sum(ArrayList<Integer> num_list, int index) {
+
+        if (num_list.size() == index) {
+            return 0;
+        } else {
+            int num = num_list.get(index);
+            return num + sum(num_list, index + 1);
+        }
+
+    }
+
+    public static void main(String[] args) {
+        int total = sum(new ArrayList<Integer>(Arrays.asList(2, 4, 6)), 0);
+        System.out.println(total);
+    }
+}