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:3Explanation:`[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 withat least`3`

citations each and the remaining two withno 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.

**Follow up:**

- This is a follow up problem to H-Index, where
`citations`

is now guaranteed to be sorted in ascending order. - Could you solve it in logarithmic time complexity?

```
struct Solution;
use std::cmp::Ordering::*;
impl Solution {
fn h_index(citations: Vec<i32>) -> i32 {
let n = citations.len();
let mut l = 0;
let mut r = n;
while l < r {
let m = l + (r - l) / 2;
match (citations[m] as usize).cmp(&(n - m)) {
Equal => {
return (n - m) as i32;
}
Less => {
l = m + 1;
}
Greater => {
r = m;
}
}
}
(n - l) as i32
}
}
#[test]
fn test() {
let citations = vec![0, 1, 3, 5, 6];
let res = 3;
assert_eq!(Solution::h_index(citations), res);
let citations = vec![1, 2];
let res = 1;
assert_eq!(Solution::h_index(citations), res);
let citations = vec![11, 15];
let res = 2;
assert_eq!(Solution::h_index(citations), res);
}
```