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# 0274. H-Index

<https://leetcode.com/problems/h-index>

## Description

Given an array of integers `citations` where `citations[i]` is the number of citations a researcher received for their `ith` paper, return compute the researcher's `h`**-index**.

According to the [definition of h-index on Wikipedia](https://en.wikipedia.org/wiki/H-index): A scientist has an index `h` if `h` of their `n` papers have at least `h` citations each, and the other `n − h` papers have no more than `h` citations each.

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

**Example 1:**

```
**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, their h-index is 3.
```

**Example 2:**

```
**Input:** citations = [1,3,1]
**Output:** 1
```

**Constraints:**

* `n == citations.length`
* `1 <= n <= 5000`
* `0 <= citations[i] <= 1000`

## ac1: sorting

O(NlogN) complexity Key: understand the definition of h-index

```java
class Solution {
    public int hIndex(int[] citations) {
        Arrays.sort(citations);

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

        return h;
    }
}
```