Seems to work

src/Balls.java

@@ -1,14 +1,31 @@
 
 public class Balls {
 
-   enum Color {green, red};
+    enum Color {green, red};
+
+    public static void main (String[] param) {
+        // for debugging
+    }
    
-   public static void main (String[] param) {
-      // for debugging
-   }
-   
-   public static void reorder (Color[] balls) {
-      // TODO!!! Your program here
-   }
+    public static void reorder (Color[] balls) {
+        int redBallCount = 0;
+
+        for (int i = 0; i < balls.length; i++) {
+            if (balls[i] == Color.red) {
+                redBallCount++;
+            }
+        }
+
+
+        if (redBallCount == balls.length) {
+            return;
+        }
+        for (int i = 0; i < redBallCount; i++) {
+            balls[i] = Color.red;
+        }
+        for (int i = redBallCount; i < balls.length; i++) {
+            balls[i] = Color.green;
+        }
+    }
 }