+---+ +---+ +---+
| 1 | --> | 4 | <-- | 3 |
+---+ +---+ +---+
^
|
|
+---+
| 2 |
+---+
+---+ +---+ +---+
| 1 | --> | 4 | <-- | 3 |
+---+ +---+ +---+
^
|
|
+---+
| 2 |
+---+