Given a **sorted** array `nums` and a `target`, return the 0-based indices `[i, j]` of the two numbers that add up to `target`. Exactly one solution exists.
Example: `twoSum([1, 2, 4, 7], 6)` → `[1, 2]`.
Hints
Start one pointer at the left end and one at the right end.
If the sum is too large, move the right pointer left; if too small, move the left pointer right.