Leetcode #147: Insertion Sort List

func insertionSortList(_ head: ListNode?) -> ListNode? {
    if head == nil {
        return nil
    }
    let dummyHead = ListNode(0)
    dummyHead.next = head
    var nextNode = head?.next
    head?.next = nil
    while nextNode != nil {
        let tmp = nextNode
        nextNode = nextNode?.next
        var pre = dummyHead
        while pre.next != nil && pre.next!.val < tmp!.val {
            if let node = pre.next {
                pre = node
            } else {
                break
            }
        }
        let node = pre.next
        pre.next = tmp
        pre.next?.next = node

    }
    return dummyHead.next
}