Two half maps

Problem

"You've got the other half," the quartermaster tells the captain, and lays her piece of the map next to his. Where one of them lost a letter, the other may still have it.

You get two strings of the same length, made of lowercase letters and the ? sign, which marks what was lost. Return the joined map, position by position: if one has ?, use the other's letter; if both have ?, it stays ?; if both have the same letter, use that letter. If at any position they have different letters, they aren't from the same treasure: return "", even if everything else fits. Two empty strings give "".

With "i?la?d" and "?s?an?" you return "island".

Examples

• They fill each other in

  "i?la?d", "?s?an?" → "island"

• A gap nobody has

  "c?e?t", "ch??t" → "che?t"

• They clash on the last letter

  "chest", "ch?sk" → ""

• Empty maps

  "", "" → ""

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def join_maps(a, b):
    pass

function joinMaps(a, b) {
}