main.kt

import java.util.Stack;

class TreeNode(var value: Int = 0) {
    var left: TreeNode? = null
    var right: TreeNode? = null
}

/*
The binary tree in this example is shown as follow

       1
     /   \
    2     3
   / \   /  
  4   5 6
     /
    7

pre-order traversal: 1 2 4 5 7 3 6
*/

fun preorderTraversalByRecursion(root: TreeNode?) {
    if (root == null) return

    print("${root.value} ")
    preorderTraversalByRecursion(root.left)
    preorderTraversalByRecursion(root.right)
}


fun preorderTraversal(root: TreeNode?) {
    if (root == null) return

    val nodeStack = Stack<TreeNode>()
    var node = root
    while (node != null || nodeStack.isNotEmpty()) {
        while (node != null) {
            print("${node.value} ")
            nodeStack.push(node.right)
            node = node.left
        }
        node = nodeStack.pop()
    }
}

fun generateBTree(nodes: Array<Int?>, index: Int): TreeNode? {
    var node: TreeNode? = null
    
    if (index < nodes.size && nodes[index] != null) {
        node = TreeNode(nodes[index]!!)
        node.left = generateBTree(nodes, index * 2 + 1)
        node.right = generateBTree(nodes, index * 2 + 2)
    }

    return node
}

fun main(args: Array<String>) {
    val nodes = arrayOf(1, 2, 3, 4 ,5, 6, null, null, null, 7)

    val root = generateBTree(nodes, 0)

    preorderTraversalByRecursion(root)
    println()

    preorderTraversal(root)
    println()
}