TreeExample.kt

 class TreeNode(var `val`: Int) {
   var left: TreeNode? = null
   var right: TreeNode? = null
 }

fun main(args: Array<String>) {
  
  val root = TreeNode(1).apply {
    left = TreeNode(2).apply { // left child of 1
      left = TreeNode(4) // left child of 2
    }
    right = TreeNode(3).apply { // right child of 1
      right = TreeNode(5).apply { // right child of 3
        left = TreeNode(6) // left child of 5
        right = TreeNode(7) // right child of 5
      }
    }
  }
}