Slug: arr-two-sum-01 · Paired reading: Chapter 15
Prompt
fn two_sum_sorted(a: &[i32], target: i32) -> Option<(usize, usize)>
— ascending slice; return indices i < j with a[i] + a[j] == target,
or None. O(n) time, O(1) extra space. Indices are Copy — use that.