(C PROGRAMMING) 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 citations each and the remaining two with no more than 3 citations each, his h-index is 3.

Test Cases: Input: [15, 10, 4] Output: 3 Input: [1, 0, 1, 0, 10] Output: 1 Input: [80, 10, 17, 123, 88, 10, 11, 19] Output: 8