21 Merge Two Sorted Lists

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

    if(NULL == l1) return l2;
    if(NULL == l2) return l1;

    ListNode* head=NULL;    // head of the list to return

    // find first element (can use dummy node to put this part inside of the loop)
    if(l1->val < l2->val)       { head = l1; l1 = l1->next; }
    else                        { head = l2; l2 = l2->next; }

    ListNode* p = head;     // pointer to form new list

    // I use && to remove extra IF from the loop
    while(l1 && l2){
        if(l1->val < l2->val)   { p->next = l1; l1 = l1->next; }
        else                    { p->next = l2; l2 = l2->next; }
        p=p->next;
    }

    // add the rest of the tail, done!
    if(l1)  p->next=l1;
    else    p->next=l2;

    return head;
}

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

        if(l1->val < l2->val) {
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
        } else {
            l2->next = mergeTwoLists(l2->next, l1);
            return l2;
        }
    }
};