is there a function to get all left node?

For example, there is a department tree,

```
root\
    node1\
        node1-1
        node1-2
    node2\
        node2-1
        node2-2
    node3\
```

is there a function to get  `node2-21` left node all? I can see all the node {root,node1,node1-1,node1-2,node2,node2-1}
Thank you.
