Binary Search tree implementation in go

BSTree.go

//A binary search tree has reference to its node
type BSTree struct {
	root *Node
}

func (btree *BSTree) isEmpty() bool {
	return btree.root == nil
}

//search for the key recursively
func get(x *Node, key string) *Node {
	if x == nil {
		return nil
	}
	if x.key > key {
		return get(x.left, key)
	} else if x.key < key {
		return get(x.right, key)
	} else {
    //x.key must equal key
		return x
	}
}

//update a key, or insert a new node into tree
func put(x *Node, key string, value int) *Node {
	if x == nil {
    //we've reached a nil left or right subtree. 
		return &Node{
			key: key,
			val: value,
		}
	}
	if x.key > key {
		x.left = put(x.left, key, value)
	} else if x.key < key {
		x.right = put(x.right, key, value)
	} else {
    //if the key already exists in the tree, update it
		x.val = value
	}
	return x
}

func returnKeys(x *Node) []Node {
	ordered := []Node{}
	if x == nil {
		return ordered
	}
	//in-order traversal
	ordered = append(ordered, returnKeys(x.left)...)
	ordered = append(ordered, *x)
	ordered = append(ordered, returnKeys(x.right)...)
	return ordered
}

type Node struct {
	key   string
	val   int
	left  *Node
	right *Node
}