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export class Node {    val: number;    left: Node | null;    right: Node | null;    next: Node | null;    constructor(val?: number, left?: Node, right?: Node, next?: Node) {        this.val = val === undefined ? 0 : val;        this.left = left === undefined ? null : left;        this.right = right === undefined ? null : right;        this.next = next === undefined ? null : next;    }}