## 275. H-Index II

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 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`.```

Note:

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

• 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?

## Rust Solution

``````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);
}
``````

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