H-Index

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. The h-index is defined as the maximum value of h such that the given author/journal has published h papers that have each been cited at least h times.

Example:

Input: citations = [10,9,8,7,3]
Output: 4 
Explanation: [10,9,8,7,4] means the researcher has 5 papers in total and each of them had 
             received 10, 9, 8, 7, 4 citations respectively. 
             Since the researcher has 4 papers with at least 4 citations each and the                 remaining one has only 3 citations i.e. less than 4, his h-index is 4.

Approach: This is a simple problem to solve. Just use sorting.

Implementation in C#:

        public static int HIndex(int[] citations)

        {

            if (citations?.Length == 0)

            {

                return 0;

            }

            Array.Sort(citations);

            Array.Reverse(citations);

            int hIndex = 0;

            for (int i = 0; i < citations.Length; ++i)

            {

                if (i + 1 > citations[i])

                {

                    break;

                }

                hIndex = Math.Max(hIndex, Math.Min(i + 1, citations[i]));

            }

            return hIndex;

        }

Complexity: O(nlogn)