深度优先搜索

class Solution {

    List<Integer> list = new LinkedList<>();

    public List<Integer> postorder(Node root) {

        if (root == null){
            return list;
        }

        for (Node c : root.children){
            postorder(c);
        }

        list.add(root.val);

        return list;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

迭代

class Solution {

    public List<Integer> postorder(Node root) {

        List<Integer> list = new LinkedList<>();
        Stack<Node> stack = new Stack<>();

        if (root == null){
            return list;
        }

        stack.push(root);

        while (!stack.isEmpty()){

            Node temp = stack.pop();
            list.add(temp.val);

            for (Node c : root.children){
                stack.push(c);
            }
        }

        Collections.reverse(list);
        
        return list;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

https://leetcode-cn.com/problems/n-ary-tree-preorder-traversal/