Binary Search — Median of Two Sorted Arrays

solve(a, b) — a and b are sorted ascending and not both empty. Return the
median of the combined values: the middle one when the total count is odd, the
average of the two middle ones when it is even. Run in O(log(min(m, n))).

Merging is O(m + n). The trick is to binary search a partition of the shorter array:
cut a after i elements and b after j = half - i elements, so the left side holds
the lowest half of all values. The cut is right when a[i-1] <= b[j] and
b[j-1] <= a[i]; use -Infinity / Infinity for a missing neighbour.

Examples

solve([1,3], [2])        → 2
solve([1,2], [3,4])      → 2.5
solve([], [1])           → 1