21.合并两个有序链表

https://leetcode-cn.com/problems/merge-two-sorted-lists/

func mergeTwoLists(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
    if l1 == nil {return l2}
    if l2 == nil {return l1}
    let dummyNode = ListNode(0)
    var cur = dummyNode
    var L1 = l1
    var L2 = l2
    
    while L1 != nil && L2 != nil {
        if L1!.val > L2!.val {
            cur.next = L2!
            cur = L2!
            L2 = L2?.next
        } else {
            cur.next = L1!
            cur = L1!
            L1 = L1?.next
        }
    }
    if L1 != nil {
        cur.next = L1
    }
    if L2 != nil {
        cur.next = L2
    }
    return dummyNode.next
}