Median of Two Sorted Arrays

You are given two arrays nums1 and nums2, each sorted in non-decreasing order (either one may be empty, but not both). Return the median of all m + n values taken together, as a float.

• If the total count is odd, the median is the middle value of the combined sorted order.
• If it is even, the median is the average of the two middle values.

The expected solution runs in O(log(m + n)) time; merging the arrays is O(m + n) and is the baseline to improve on. Answers within 10^-9 of the expected value are accepted.