Given the root of an n-ary tree, return the postorder traversal of its nodes’ values.

Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)

Example 1:

Input: root = [1,null,3,2,4,null,5,6]
Output: [5,6,3,2,4,1]

Example 2:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [2,6,14,11,7,3,12,8,4,13,9,10,5,1]

<解題>

class Solution {
    public List<Integer> postorder(Node root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        postorder(root, res);
        return res;
    }
    
    private void postorder(Node node, List<Integer> res) {
        for (Node c : node.children) {
            postorder(c, res); 
        }
        res.add(node.val);
    }
}

Time: O(n)

Space: O(h)