6-2 两个有序链表序列的合并

合并两个有序链表的方法如下所示：

def mergeTwoLists(l1, l2):
    dummy = ListNode(0)  # 创建一个虚拟头结点
    curr = dummy  # 创建一个指针指向虚拟头结点
    while l1 and l2:
        if l1.val <= l2.val:
            curr.next = l1
            l1 = l1.next
        else:
            curr.next = l2
            l2 = l2.next
        curr = curr.next
    curr.next = l1 if l1 else l2
    return dummy.next