Sorted Two-Sum: Mental Model

Start with the idea

Begin with the concrete question: Return one-based indexes of the pair summing to 17. For the visible input [2,3,7,11,15], the stated result is 1,5. Before reading code, name the smallest piece of information that must survive from one step to the next. Two positions move according to a monotone fact: after one comparison, some candidates can be ruled out permanently. Name that fact. That is the mental model for Sorted Two-Sum; it gives you a reason for each update instead of a line of code to memorise.

The core mechanism is this: Compare the end-pair sum with the target and discard one impossible endpoint. 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 a starting point. Identify what it stores, how one iteration changes it, and whether it already solves the exact task or only demonstrates a related operation.