Merge Two Sorted Lists

ListNode *mergeList(ListNode *l1, ListNode *l2)
{
if (l1 == nullptr)return l2;
if (l2 == nullptr)return l1;

ListNode dummy(-1);
ListNode *p = &dummy;

for (; l1 != nullptr&&l2 != nullptr; p = p->next)
{
if (l1->val > l2->val)
{
p->next = l2;
l2 = l2->next;
}
else
{
p->next = l1;
l1 = l1->next;
}
}

p->next = l1 != nullptr ? l1 : l2;
return dummy.next;
}