H-Index
<p>Given an array of integers <code>citations</code> where <code>citations[i]</code> is the number of citations a researcher received for their <code>i<sup>th</sup></code> paper, return <em>the researcher&#39;s h-index</em>.</p>

<p>According to the <a href="https://en.wikipedia.org/wiki/H-index" target="_blank">definition of h-index on Wikipedia</a>: The h-index is defined as the maximum value of <code>h</code> such that the given researcher has published at least <code>h</code> papers that have each been cited at least <code>h</code> times.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> citations = [3,0,6,1,5]
<strong>Output:</strong> 3
<strong>Explanation:</strong> [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.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> citations = [1,3,1]
<strong>Output:</strong> 1
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == citations.length</code></li>
	<li><code>1 &lt;= n &lt;= 5000</code></li>
	<li><code>0 &lt;= citations[i] &lt;= 1000</code></li>
</ul>
Array; Counting Sort; Sorting
impl Solution {
    #[allow(dead_code)]
    pub fn h_index(citations: Vec<i32>) -> i32 {
        let mut citations = citations;
        citations.sort_by(|&lhs, &rhs| rhs.cmp(&lhs));

        let n = citations.len();

        for i in (1..=n).rev() {
            if citations[i - 1] >= (i as i32) {
                return i as i32;
            }
        }

        0
    }
}
