589. N叉树的前序遍历及后序遍历（递归实现）

前序遍历，（先添加当前结点到结果llist，再遍历当前结点的children list。）

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
    List<Integer> list = new ArrayList<Integer>();
    public List<Integer> preorder(Node root) {
        if (root == null) 
            return list;
        list.add(root.val);
        for (Node node : root.children)
            preorder(node);
        return list;
    }
}

后续遍历（先遍历当前结点的children list。再添加当前结点到结果list）

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