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## 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.”

Example:

Input: citations = [3,0,6,1,5]
Output: 3
Explanation: [3,0,6,1,5] 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, her h-index is 3.


Note: If there are several possible values for h, the maximum one is taken as the h-index.

## Code

class Solution {
public int hIndex(int[] citations) {
if(citations == null || citations.length == 0) return 0;

Arrays.sort(citations);

int count = 1;

for(int i = citations.length - 1; i >= 0; i--){
if(count <= citations[i]){
count++;
} else {
return count - 1;
}
}

return count - 1;
}
}

class Solution:
def hIndex(self, citations: List[int]) -> int:
citations.sort(reverse=True)

res = 0
for i in range(len(citations)):
if i + 1 <= citations[i]:
res = i + 1

return res