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.

Solution:
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *head = NULL, *trav = NULL;
        while (l1 != NULL && l2 != NULL) {
            if (l1 -> val < l2 -> val) {
                if (trav == NULL) {
                    trav = l1;
                    head = l1;
                } else {
                    trav -> next = l1;
                    trav = l1;
                }
                l1 = l1 -> next;
            } else {
                if (trav == NULL) {
                    trav = l2;
                    head = l2;
                } else {
                    trav -> next = l2;
                    trav = l2;
                }
                l2 = l2 -> next;
            }
        }
        if (trav != NULL)
            trav -> next = (l1 != NULL ? l1 : l2);
        else
            head = (l1 != NULL ? l1 : l2);
        return head;
    }
};