590 - N-ary Tree Postorder Traversal

 JAVA

class Solution {
    public List<Integer> postorder(Node root) {
        LinkedList<Integer> ans = new LinkedList<>();
        if (root == null) {
            return ans;
        }
        Deque<Node> stk = new ArrayDeque<>();
        stk.offer(root);
        while (!stk.isEmpty()) {
            root = stk.pollLast();
            ans.addFirst(root.val);
            for (Node child : root.children) {
                stk.offer(child);
            }
        }
        return ans;
    }
}

C++

class Solution {
public:
    vector<int> postorder(Node* root) {
        vector<int> ans;
        if (!root) return ans;
        stack<Node*> stk{ {root} };
        while (!stk.empty()) {
            root = stk.top();
            ans.push_back(root->val);
            stk.pop();
            for (Node* child : root->children) stk.push(child);
        }
        reverse(ans.begin(), ans.end());
        return ans;
    }
};