Team Photo I

You're photographing two teams arranged in two rows. Every back-row player must be strictly taller than the front-row player directly in front of them.

Two teams of equal size each fill one row, and you may order each row however you like. Return which team goes in front: ["A", "B"] when team A can stand in front of team B, ["B", "A"] when team B can stand in front of team A, and null when neither works.

teamA = [170, 160], teamB = [180, 165]   ->  ["A", "B"]   (160 before 165, 170 before 180)
teamA = [3, 4],     teamB = [1, 2]       ->  ["B", "A"]
teamA = [170],      teamB = [170]        ->  null         (equal height blocks)