Given a 1-indexed sorted array, find two numbers that add up to a target. Return their indices (1-based).
Since the array is sorted, we can use two pointers instead of a hash map. This gives us O(1) space instead of O(n).
Why two pointers work on sorted arrays: if the sum is too small, moving left pointer right increases it. If too large, moving right pointer left decreases it.
two_sum_ii([2, 7, 11, 15], 9)
[1, 2]