The missing band

Problem

At dawn, the biologist walks around the lagoon with her binoculars. A month ago she put a leg band on each heron, numbered from 1 to n, a different one for each bird, and today all of them came back but one. Write a function that takes seen, the list of bands she counted today (integers with no repeats, in any order), and returns an integer: the missing number. Since exactly one heron is missing, n is the length of the list plus one. The missing one can be any of them, including band 1 or band n. If the list is empty, there was only one heron: 1 is missing. With [3, 1, 5, 2], n is 5. The bands from 1 to 5 add up to 15, the ones seen add up to 11, so 4 is missing. There is no need to look for each number one by one: compare what they should add up to with what they do.

Examples

• The example

  [3, 1, 5, 2] → 4

• One from the middle is missing

  [2, 4, 1] → 3

• Band 1 is missing

  [2] → 1

• Seven herons

  [6, 2, 7, 1, 3, 4] → 5

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

You start with this

def missing_band(seen):
    pass

function missingBand(seen) {
}