+-------------------+
| v
+---+ +---+ +---+
| 1 | --> | 2 | --> | 3 |
+---+ +---+ +---+
+---+----+
| 3 | 4 |
+---+----+
+---+----+----+
| 5 | 6 | 7 |
+---+----+----+
+-------------------+
| v
+---+ +---+ +---+
| 1 | --> | 2 | --> | 3 |
+---+ +---+ +---+
+---+----+
| 3 | 4 |
+---+----+
+---+----+----+
| 5 | 6 | 7 |
+---+----+----+