A binary tree is a tree that includes at most two children, often referred
to as the left and right children:

Kotlin Implementation 

class BinaryNode<T>(
    val value: T,
    var left: BinaryNode<T>? = null,
    var right: BinaryNode<T>? = null
)

Binary tree traversing

We can traverse the binary tree in three ways.

• pre-order traversal
• in-order traversal
• post-order traversal

Pre-order traversal

In the preorder traversal algorithm, we first visit the root node and then
the left and right node recursively.

kotlin Extention function for pre-order traversal algorithm

fun <T> BinaryNode<T>.preOrder( visit: (BinaryNode<T>) -> Unit) {
    visit(this)
    left?.preOrder(visit)
    right?.preOrder(visit)
}
  

In-order traversal

In order-traversal visit the node in the following order.

• if the current node has a left child, recursively visit it first.
• visit the current node itself
• if the current node has a right child, recursively visit this child.

 
 
 fun <T> BinaryNode<T>.inOrder( visit: (BinaryNode<T>) -> Unit) {
    left?.inOrder(visit)
    visit(this)
    right?.inOrder(visit)
 }
 
 

Post-order traversal

The post order traversal visits the node in the following order.

• if the current node has a left child, recursively visit this child first.
• if the current node has the right child, recursively visit this child.
• visit the node itself

fun <T> BinaryNode<T>.postOrder(visit: (BinaryNode<T>) -> Unit) {
    left?.postOrder(visit)
    right?.postOrder(visit)
    visit(this)
}
 

let's create a tree and traverse it with all three functions

fun main() {
    val one = BinaryNode(1)
    val two = BinaryNode(2)
    val three = BinaryNode(3)
    val four = BinaryNode(4)
    val five = BinaryNode(5)
    val six = BinaryNode(6)

    one.left = two
    one.right = three

    two.left = four
    two.right = five

    three.right = six

    println("Pre order traversal: ")
    one.preOrder {
        print(it.value.toString() + ", ")
    }

    println("nIn order traversal: ")
    one.inOrder {
        print(it.value.toString() + ", ")
    }

    println("nPost order traversal")
    one.postOrder {
        print(it.value.toString() + ", ")
    }
}
  

Output

Pre order traversal: 
1, 2, 4, 5, 3, 6, 
In order traversal: 
4, 2, 5, 1, 3, 6, 
Post order traversal
4, 5, 2, 6, 3, 1,