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) {
        val = _val;
    }

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

class Solution {
    List res=new ArrayList();
    public List<Integer> preorder(Node root) {
        if(root==null) return res;
        help(root);
        return res;
    }
   private void help(Node node){
       if(node==null) return ;
       res.add(node.val);

       if(node.children!=null){
           int s=node.children.size();
           for(int i=0;i<s;i++){
               help(node.children.get(i));
           }
       }
       
   }
}