21. Merge Two Sorted Lists

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* temp= new ListNode(0);
ListNode** res = &(temp->next);
while (l1&&l2) {
if (l1->val <= l2->val) {
temp->next = l1;
l1 = l1->next;
}
else if (l2->val < l1->val) {
temp->next = l2;
l2 = l2->next;
}
temp = temp->next;
}
temp->next = l1 ? l1 : l2;
return *res;
}