Merge Nodes in Between Zeros

Consume delimiter-separated groups

The standard input begins and ends with zero and every interior group contains positive values. Walk from the node after the leading zero, accumulating values. On reaching a zero, append one output node holding the accumulated sum, reset the sum, and continue. A dummy output head makes appending simple and avoids a special case for the first group. You can reuse a group-start node in variants that permit mutation, but allocating output nodes is easier to reason about.

Validate assumptions

Say that the delimiter contract matters: consecutive zeros would create an empty group unless the statement rules them out. Do not include delimiter values in the sum, and do not emit a trailing extra node after the final zero. Each input node is visited once, so time is O(n); the produced list costs O(g) space for g groups, while working space is O(1). Test one group, many single-value groups, and the minimum delimiter-only shape if allowed.