Challenge: Merge Two Sorted Lists

Start with the idea

Solve the exact task, not just the visible example: Return the merged sorted values. On input 1->2->4 and 1->3->4, predict the output before opening the answer reveal. The revealed output is a test oracle for this case, while private checks exercise other legal cases. Start by stating the input grammar and output format in your own words. Parse the complete input, including surrounding whitespace, before relying on any convenient positions in the example.

Build the solution around one maintained claim: Always detach the smaller head and append it to the merged tail. Decide what state represents that claim, how it is initialized, what a single update does, and when the algorithm stops. A list algorithm changes relationships between nodes, not just values. Draw the next pointers before and after each mutation. Trace the visible input by hand and record at least two intermediate states, not only the final answer. If the trace cannot justify your predicted output, there is a gap between the idea and the implementation.

Use the Rust sketch as a guide, not a substitute for a complete solve function. The supplied Rust sketch is an invariant note rather than a complete program. Turn it into explicit state and transitions before coding. Rust ownership makes linked structures explicit: use the provided node type, take links carefully, and preserve access to the remaining chain.