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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

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;
        }
    }
};

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode head(0);
        ListNode *tail = &head;

        while(l1 && l2){
            if(l1 -> val < l2 -> val){
                tail -> next = l1;
                l1 = l1 -> next;
            }else{
                tail -> next = l2;
                l2 = l2 -> next;
            }
            tail = tail -> next;
        }
        tail -> next = l1 ? l1 : l2;
        return head.next;
    }
};