**Problem:**

Given an array of citations (each citation is a non-negative integer) of a researcher, write a function to compute the researcher’s h-index.

According to the definition of h-index on Wikipedia: “A scientist has index *h* if *h* of his/her *N* papers have **at least** *h* citations each, and the other *N − h* papers have **no more than** *h* citations each.”

For example, given `citations = [3, 0, 6, 1, 5]`

, which means the researcher has `5`

papers in total and each of them had received `3, 0, 6, 1, 5`

citations respectively. Since the researcher has `3`

papers with **at least** `3`

citations each and the remaining two with **no more than** `3`

citations each, his h-index is `3`

.

**Note**: If there are several possible values for `h`

, the maximum one is taken as the h-index.

**Hint:**

- An easy approach is to sort the array first.
- What are the possible values of h-index?
- A faster approach is to use extra space.

**Solution:**

public class Solution { public int hIndex(int[] citations) { Arrays.sort(citations); int h = citations.length; for (int i = 0; i < citations.length; i++) { if (citations[i] >= h) return h; h--; } return 0; } }