Similar to array, the difference is if any of two listnode is not null after first loop, we only need to add it as previous node's next and no need to add them one by one.

```
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null && l2 == null) {
return null;
}
if (l1 == null) {
return l2;
}
if (l2 == null) {
return l1;
}
ListNode result = new ListNode(0);
ListNode prev = result;
while (l1 != null && l2 != null) {
if (l1.val <= l2.val) {
prev.next = l1;
l1 = l1.next;
} else {
prev.next = l2;
l2 = l2.next;
}
prev = prev.next;
}
if (l1 != null) {
prev.next = l1;
}
if (l2 != null) {
prev.next = l2;
}
return result.next;
}
}
```