public:

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {

ListNode* head = new ListNode(0);

ListNode* tail = head;

while (l1 != NULL || l2 != NULL) {

// Use l1 if l2 is NULL (then we would know that l1 has to be non-null,

// since the while loop would exit otherwise), or if l1 is != NULL and

// l1 has a value less than l2 (we know l2 can be safely called because

// it passed the first check and failed to short circuit).

//

// Otherwise, use l2.

if (l2 == NULL || (l1 != NULL && l1->val < l2->val)) {

tail->next = l1;

l1 = l1->next;

} else {

tail->next = l2;

l2 = l2->next;

}

tail = tail->next;

}

return head->next;

}