Решение

class Solution {
    fun postorder(root: Node?): List<Int> {
        if (root == null) return emptyList()
        var cap = 1024
        var stack = arrayOfNulls<Node>(cap)
        var idx = IntArray(cap)
        var top = 0
        fun push(x: Node) {
            if (top == cap) {
                val ncap = cap shl 1
                val ns = arrayOfNulls<Node>(ncap)
                System.arraycopy(stack, 0, ns, 0, cap)
                stack = ns
                idx = idx.copyOf(ncap)
                cap = ncap
            }
            stack[top] = x
            idx[top] = 0
            top++
        }
        push(root)
        val res = ArrayList<Int>()
        while (top > 0) {
            val t = top - 1
            val node = stack[t]!!
            val i = idx[t]
            val ch = node.children
            if (i < ch.size) {
                idx[t] = i + 1
                val c = ch[i]
                if (c != null) push(c)
            } else {
                res.add(node.`val`)
                stack[t] = null
                top--
            }
        }
        return res
    }
}