Since we know both input lists are in non-decreasing order. Basically, we just compare on two lists and "pick" the smaller one to form the answer.

```
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if(l1 == null)
return l2;
if(l2 == null)
return l1;
ListNode fakeHead = new ListNode(-1);
ListNode p = fakeHead;
while(l1 != null && l2 != null){
if(l1.val <= l2.val){
p.next = l1;
l1 = l1.next;
}else{
p.next = l2;
l2 = l2.next;
}
p = p.next;
}
if(l1 == null)
p.next = l2;
if(l2 == null)
p.next = l1;
return fakeHead.next;
}
```