Copy with Random Links: Mental Model

Start with the idea

Begin with the concrete question: Return copied values with random target indexes. For the visible input values=[7,13,11] random=[null,0,1], the stated result is 7:null,13:0,11:1. Before reading code, name the smallest piece of information that must survive from one step to the next. A list algorithm changes relationships between nodes, not just values. Draw the next pointers before and after each mutation. That is the mental model for Copy with Random Links; it gives you a reason for each update instead of a line of code to memorise.

The core mechanism is this: Map old nodes to new nodes or interleave copies so arbitrary links can be reconstructed. Treat it as a claim to test after every operation. Write down the state before the first operation, update it once, and ask which part of the claim is still true. If an item is skipped, inserted, or removed, explain why that action cannot destroy an answer that should be retained. A useful trace has columns for the current input position, the state before the step, the action, and the state afterward. Use the visible sample until the expected result becomes inevitable rather than merely plausible.

Now inspect the Rust sketch, not just its final expression. The supplied Rust sketch is an invariant note rather than a complete program. Turn it into explicit state and transitions before coding.