/*
 * Iterative approach
 */
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
    struct ListNode *list = NULL;
    struct ListNode *tmp = NULL;

    if (!l1)
        return l2;
    if (!l2)
        return l1;

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

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

        if (l1)
            tmp->next = l1;
        if (l2)
            tmp->next = l2;

        return list;
    }

    return NULL;
}

/*
 * Recursive approach
 */
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
    if(!l1)
        return l2;
    if(!l2)
        return l1;
    if(l1->val < l2->val) {
        l1->next = mergeTwoLists(l1->next, l2);
        return l1;
    } else {
        l2->next = mergeTwoLists(l1, l2->next);
        return l2;
    }
}