Given two non-empty lists of integers a and b, both sorted in non-decreasing order, pick one value from each so that the closed span between them is as short as possible. Return that span as [lo, hi] (lo <= hi). If several spans have the same minimal length, return the one with the smallest lo.
Examples
Input: a = [1, 4, 10], b = [6, 7, 12]
Output: [4, 6]
Explanation: [4, 6] and [10, 12] both have length 2; the smaller lo wins.
Input: a = [5], b = [1, 9]
Output: [1, 5]
Constraints
1 <= len(a), len(b) <= 10**5- Target: O(len(a) + len(b)) time, O(1) extra space.
Goals
- Walk two sorted lists like a merge and examine each cross-list neighbouring pair
- Apply a tie-break consistently while scanning