Pair Sum in a Sorted List

A shop lists its prices from cheapest to dearest, and some prices repeat. Write pair_sum_sorted(prices, target) that finds two different items whose prices add up to exactly target and returns their prices as [cheaper, dearer]. If several pairs work, return the one whose cheaper item is cheapest. If no pair works, return [].

Two items can have the same price, but one item cannot be used twice.

Examples

pair_sum_sorted([2, 3, 5, 6, 8, 9], 11)   # returns [2, 9]
pair_sum_sorted([1, 4, 4, 9], 8)          # returns [4, 4]
pair_sum_sorted([3, 4, 9], 8)             # returns []

In the first example 2 + 9, 3 + 8 and 5 + 6 all make 11, and the pair with 2 has the cheapest item. In the last example 4 + 4 would make 8, but there is only one item priced 4.

Constraints

• 0 <= len(prices) <= 100,000
• 0 <= prices[i], target <= 10^9
• prices is sorted in ascending order