428.Serialize and Deserialize N-ary Tree

// Idea: preorder recursive traversal; add number of children after root val,in order to know when to terminate.
// Example: The example in description is serialized as: "1,3,2,5,6,4,0"

class Codec {

    // Encodes a tree to a single string.
    public String serialize(Node root) {
        List<String> list=new LinkedList<>();
        serializeHelper(root,list);
        return String.join(",",list);
    }
    
    private void serializeHelper(Node root,List<String> list){
        if(root==null){
            return;
        }else{
            list.add(String.valueOf(root.val));
            list.add(String.valueOf(root.children.size()));
            for(Node child:root.children){
                serializeHelper(child,list);
            }
        }
    }

    // Decodes your encoded data to tree.
    public Node deserialize(String data) {
        if(data.isEmpty())
            return null;
        
        String[] ss=data.split(",");
        Queue<String> q=new LinkedList<>(Arrays.asList(ss));
        return deserializeHelper(q);
    }
    
    private Node deserializeHelper(Queue<String> q){
        Node root=new Node();
        root.val=Integer.parseInt(q.poll());
        int size=Integer.parseInt(q.poll());
        root.children=new ArrayList<Node>(size);
        for(int i=0;i<size;i++){
            root.children.add(deserializeHelper(q));
        }
        return root;
    }
}