PostOrderTraversalIterative.kt

import java.util.ArrayDeque
import java.util.LinkedList

fun postOrderIterative(root: TreeNode?): List<Int> {
    if(root == null) return emptyList()
    val stack = ArrayDeque<TreeNode>()
    val list = LinkedList<Int>()
    stack.push(root)
    while (stack.isNotEmpty()) {
        val node = stack.pop()
        list.addFirst(node.data)
        node.left?.let { stack.push(it) }
        node.right?.let { stack.push(it) }
    }
    return list
}