const flatten = (head) => {
    if (!head) {
        return head
    }

    let result = new Node(head.val, null, null, null)
    let resultTail = result

    const handleChild = (node) => {
        if (node.child) {
            handleNode(node.child)
        }
    }

    const handleNode = (node) => {
        if (!node) {
            return
        }
        resultTail.next = new Node(node.val, resultTail, null, null)
        resultTail = resultTail.next
        handleChild(node)
        handleNode(node.next)
    }

    handleChild(head)
    handleNode(head.next)

    return result
}