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