The researcher's score

Problem

The observatory's committee is handing out grants, and it doesn't want to reward only those who publish a lot, nor those with a single famous paper. So it grades each researcher with a score that asks for both. Write a function that takes citations, a list of integers from 0 up (how many times each of her papers was cited; it may be empty), and returns an integer: the largest number h such that she has at least h papers with h or more citations each.

With [3, 0, 6, 1, 5] the score is 3: three papers have 3 citations or more (the 3, the 6 and the 5). It doesn't reach 4, because only two have 4 or more. A single paper with 100 citations gives 1, not 100. With no papers, or if none of them was cited, the score is 0.

Examples

• The one from the example

  [3, 0, 6, 1, 5] → 3

• Four equal papers

  [4, 4, 4, 4] → 4

• One heavily cited paper doesn't raise the score

  [25, 8, 5, 3, 3] → 3

• Nobody cited them

  [0, 0, 0] → 0

• No papers

  [] → 0

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

You start with this

def score(citations):
    pass

function score(citations) {
}