When are we both free?

Two people each give you the window they are free in, as minutes counted from midnight. A window is a pair [start, end] and it includes both ends. You want the window in which both of them are free, which is the intersection of the two intervals.

Return that shared window as a pair [start, end], or an empty list if there is no moment they are both free.

Worked example

Input: a = [540, 720], b = [660, 900]

The shared start is the later of the two starts: max(540, 660) = 660
The shared end   is the earlier of the two ends:  min(720, 900) = 720
660 is not after 720, so the window is real.

Output: [660, 720]

The awkward ends

Input: a = [0, 100],   b = [200, 300]   Output: []
Input: a = [100, 200], b = [120, 150]   Output: [120, 150]
Input: a = [100, 200], b = [200, 300]   Output: [200, 200]

Constraints

• a and b each hold exactly two values, with the start no later than the end.
• Values run from $-10^{9}$ to $10^{9}$, so they need not be minutes of a real day, and they may be negative.
• Windows that touch at a single value do overlap, at that one value, and the answer is a window of zero width.
• Return an empty list, not a pair, when there is no overlap.