21_Merge Two Sorted Lists

/*

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

*/

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    if (!l1) return l2;
    if (!l2) return l1;

    ListNode ans(0);
    ListNode *head(&ans);
    ListNode *tail(&ans);

    while (l1 && l2) {
        if (l1->val < l2->val) {
            tail->next = l1;
            tail = tail->next;
            l1 = l1->next;
        }
        else
        {
            tail->next = l2;
            tail = tail->next;
            l2 = l2->next;
        }
    }

    tail->next = l1 ? l1 : l2;
    return head->next;
}
};