1669. Merge In Between Linked Lists 

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class Solution {
    public ListNode mergeInBetween(ListNode list1, int a, int b, ListNode list2) {
        ListNode curr = list1;
        for (int i = 0; i < a - 1; i++) {
            curr = curr.next;
        }
        ListNode prev = curr;
        for (int i = a - 1; i < b; i++) {
            curr = curr.next;
        }
        ListNode next = curr.next;

        prev.next = list2;
        curr = list2;
        while (curr.next != null) {
            curr = curr.next;
        }
        curr.next = next;

        return list1;
    }
}

References

1669. Merge In Between Linked Lists