Get a list of all trees root

If we have the following data stored:

```
+--+--+  +--+--+  
|  1  |  |  3  |  
+--+--+  +--+--+  
   |        |     
+--+--+  +--+--+  
|  2  |  |  4  |  
+-----+  +-----+  
```

Is there some way to get a list with 1 and 3, the root of both trees?