無(wú)法添加新元素（節(jié)點(diǎn)）到ArrayListNode N = new Node(5,"Sandeep");Node N1 = new Node(5,"qwert");在下面的行中我收到空指針異常N.children.add(N1)代碼：class Node {    public int val;    public String data;    public ArrayList<Node> children;    public Node(int val, String data) {        this.val = val;        this.data = data;        ArrayList<Node> children = new ArrayList<Node>();    }}public class Nary {    public static void main(String[] args) {                   // ArrayList<Node> children = new ArrayList<Node>();        Node N = new Node(5,"Sandeep");        Node N1 = new Node(5,"qwert");        N.children.add(N1);    }}