Python3 two pointers commented
python\ndef mergeInBetween(self, list1: ListNode, a: int, b: int, list2: ListNode) -> ListNode:\n        pos, a_node = 0, list1\n        while pos < a - 1:     
```python\ndef mergeInBetween(self, list1: ListNode, a: int, b: int, list2: ListNode) -> ListNode:\n        pos, a_node = 0, list1\n        while pos < a - 1:          # let a_node point to the list1 node at index a - 1\n            a_node = a_node.next\n            pos += 1\n        b_node = a_node\n        while pos < b + 1:          # let b_node point to the list1 node at index b + 1\n            b_node = b_node.next\n            pos += 1\n        a_node.next = list2         # put list2 after a_node\n        while list2.next:\n            list2 = list2.next\n        list2.next = b_node         # put list2 before b_node\n        return list1                # return the head\n```
['Python', 'Python3']