Merge In Between Linked Lists

Preserve both splice boundaries

Walk the first list to the node just before index a, and separately keep the node just after index b. Find the tail of the second list by walking it once. Then set beforeA.next = list2Head and list2Tail.next = afterB. The removed segment becomes unreachable and no new nodes are needed. Draw the pointers before coding: predecessor, removed range, successor, inserted head, inserted tail.

Index safety

Most mistakes are off by one. If indices are zero-based, advance a-1 times for the predecessor and continue through b to obtain the successor. Clarify whether a can be zero; if so a dummy head makes predecessor handling uniform. Also clarify whether the second list is guaranteed nonempty. The algorithm is O(n+m) time for list lengths and O(1) extra space. Do not reconnect the predecessor directly to successor before saving it, or the insertion location is lost.