589. N叉树的前序遍历

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

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    public List<Integer> preorder(Node root) {
        List<Integer> res = new ArrayList<Integer>();
        preOrder(res,root);
        return res;
    }
    public void preOrder(List<Integer> res, Node root) {
        if(root == null) return;
        res.add(root.val);
        for(int i=0; i<root.children.size(); i++) {
            preOrder(res,root.children.get(i));
        }
    }
}