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## Problem

Given an array of citations sorted in ascending order (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 = [0,1,3,5,6]
Output: 3
Explanation: [0,1,3,5,6] means the researcher has 5 papers in total and each of them had
received 0, 1, 3, 5, 6 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.


## Code

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

int len = citations.length;

int left = 0;
int right = citations.length - 1;

while(left + 1 < right){
int mid = left + (right - left) / 2;
if(citations[mid] == len - mid){
return len - mid;
} else if(citations[mid] > len - mid){
right = mid;
} else{
left = mid;
}
}

if(citations[left] >= len - left){
return len - left;
}

if(citations[right] >= len - right){
return len - right;
}

return 0;
}
}

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

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

return count - 1;

}
}

class Solution:
def hIndex(self, citations: List[int]) -> int:
if not citations:
return 0

start = 0
end = len(citations) - 1

while start + 1 < end:
mid = (start + end) // 2

if citations[mid] == len(citations) - mid:
return len(citations) - mid
elif citations[mid] > len(citations) - mid:
end = mid
else:
start = mid

if citations[start] >= len(citations) - start:
return len(citations) - start

if citations[end] >= len(citations) - end:
return len(citations) - end

return 0

class Solution:
def hIndex(self, citations: List[int]) -> int:
count = 1
res = 0
for i in range(len(citations) - 1, -1, -1):
if count <= citations[i]:
res = count

count += 1

return res