InOrderIterative.kt

import java.util.ArrayDeque

fun inorderTraversalIterative(root: TreeNode?): List<Int> {
    val list = mutableListOf<Int>()
    if (root == null) return list
    var node = root
    val stack = ArrayDeque<TreeNode>()
//        traversing the tree whenever right node is not null or the stack contains items
    while (node != null || stack.isNotEmpty()) {
//            processing all the left nodes of the current node
        if (node != null) {
            stack.push(node) 
            node = node.left //traversing to left node without processing root data
        } else {
            node = stack.pop()
            list.add(node.data) // adding to the list if no left child
            node = node.right // processing the right subtree
        }
    }
    return list
}