Add comments

src/Balls.java

@@ -10,6 +10,7 @@ public class Balls {
     public static void reorder (Color[] balls) {
         int redBallCount = 0;
 
+        // count how many red balls there is
         for (int i = 0; i < balls.length; i++) {
             if (balls[i] == Color.red) {
                 redBallCount++;
@@ -17,9 +18,14 @@ public class Balls {
         }
 
 
+        // If all balls are red or green then just return
         if (redBallCount == balls.length) {
             return;
+        } else if (redBallCount == 0) {
+            return;
         }
+
+        // replace the contents of the array with new colors that are in correct order
         for (int i = 0; i < redBallCount; i++) {
             balls[i] = Color.red;
         }