This is my first version that can pass the OJ:

```
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if(l1 == null) return l2;
if(l2 == null) return l1;
ListNode curr = new ListNode((l1.val < l2.val ? l1.val : l2.val));
ListNode head = curr;
ListNode index1 = l1.val < l2.val ? l1.next : l1;
ListNode index2 = l1.val < l2.val ? l2: l2.next;
while(index1 != null && index2 != null) {
if(index1.val < index2.val) {
ListNode temp = new ListNode(index1.val);
curr.next = temp;
curr = temp;
index1 = index1.next;
} else {
ListNode temp = new ListNode(index2.val);
curr.next = temp;
curr = temp;
index2 = index2.next;
}
}
if(index1 != null) curr.next = index1;
if(index2 != null) curr.next = index2;
return head;
}
}
```

Then I tried to refine the code by putting the similar blocks into a function. However, it did not work and I got "Time Limit Exceeded". The updated code is:

```
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if(l1 == null) return l2;
if(l2 == null) return l1;
ListNode curr = new ListNode((l1.val < l2.val ? l1.val : l2.val));
ListNode head = curr;
ListNode index1 = l1.val < l2.val ? l1.next : l1;
ListNode index2 = l1.val < l2.val ? l2: l2.next;
while(index1 != null && index2 != null) {
if(index1.val < index2.val) {
addNode(index1, curr);
} else {
addNode(index2, curr);
}
}
if(index1 != null) curr.next = index1;
if(index2 != null) curr.next = index2;
return head;
}
public void addNode(ListNode nodeToBeAdded, ListNode curr) {
ListNode temp = new ListNode(nodeToBeAdded.val);
curr.next = temp;
curr = temp;
nodeToBeAdded = nodeToBeAdded.next;
}
}
```

Can anyone tell what is the error in the second version?