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 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.

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?

275. H-Index II
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);
}