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:3Explanation:`[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 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.

```
struct Solution;
impl Solution {
fn h_index(citations: Vec<i32>) -> i32 {
let n = citations.len();
let mut count: Vec<usize> = vec![0; n + 1];
for c in citations {
let i = c as usize;
if i < n {
count[i] += 1;
} else {
count[n] += 1;
}
}
let mut h = n;
let mut sum = 0;
while sum <= h {
sum += count[h];
if sum >= h {
return h as i32;
}
h -= 1;
}
0
}
}
#[test]
fn test() {
let citations = vec![3, 0, 6, 1, 5];
let res = 3;
assert_eq!(Solution::h_index(citations), res);
}
```