```
/**
* 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) {
// first check if either list is empty
if(l1==NULL) return l2;
if(l2==NULL) return l1;
// if both lists contain at least 1 element, establish "start" element for result
ListNode* start = NULL;
if(l1->val < l2->val) {
start = l1;
l1 = l1->next;
}
else {
start = l2;
l2 = l2->next;
}
// use a pointer to loop thru heads of both lists in order
ListNode* curr = start;
while(l1!=NULL && l2!=NULL) {
if(l1->val < l2->val) {
curr->next = l1;
l1 = l1->next;
}
else {
curr->next = l2;
l2 = l2->next;
}
curr = curr->next;
}
// until this point, either l1 or l2 should be empty, one last operation to link the remaining one to tail
if(l1!=NULL) curr->next = l1;
if(l2!=NULL) curr->next = l2;
return start;
}
};
```