Longest Cycle in a Graph
<p>You are given a <strong>directed</strong> graph of <code>n</code> nodes numbered from <code>0</code> to <code>n - 1</code>, where each node has <strong>at most one</strong> outgoing edge.</p>

<p>The graph is represented with a given <strong>0-indexed</strong> array <code>edges</code> of size <code>n</code>, indicating that there is a directed edge from node <code>i</code> to node <code>edges[i]</code>. If there is no outgoing edge from node <code>i</code>, then <code>edges[i] == -1</code>.</p>

<p>Return <em>the length of the <strong>longest</strong> cycle in the graph</em>. If no cycle exists, return <code>-1</code>.</p>

<p>A cycle is a path that starts and ends at the <strong>same</strong> node.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/2300-2399/2360.Longest%20Cycle%20in%20a%20Graph/images/graph4drawio-5.png" style="width: 335px; height: 191px;" />
<pre>
<strong>Input:</strong> edges = [3,3,4,2,3]
<strong>Output:</strong> 3
<strong>Explanation:</strong> The longest cycle in the graph is the cycle: 2 -&gt; 4 -&gt; 3 -&gt; 2.
The length of this cycle is 3, so 3 is returned.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/2300-2399/2360.Longest%20Cycle%20in%20a%20Graph/images/graph4drawio-1.png" style="width: 171px; height: 161px;" />
<pre>
<strong>Input:</strong> edges = [2,-1,3,1]
<strong>Output:</strong> -1
<strong>Explanation:</strong> There are no cycles in this graph.
</pre>

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

<ul>
	<li><code>n == edges.length</code></li>
	<li><code>2 &lt;= n &lt;= 10<sup>5</sup></code></li>
	<li><code>-1 &lt;= edges[i] &lt; n</code></li>
	<li><code>edges[i] != i</code></li>
</ul>
Depth-First Search; Breadth-First Search; Graph; Topological Sort
impl Solution {
    pub fn longest_cycle(edges: Vec<i32>) -> i32 {
        let n = edges.len();
        let mut vis = vec![false; n];
        let mut ans = -1;

        for i in 0..n {
            if vis[i] {
                continue;
            }
            let mut j = i as i32;
            let mut cycle = Vec::new();

            while j != -1 && !vis[j as usize] {
                vis[j as usize] = true;
                cycle.push(j);
                j = edges[j as usize];
            }

            if j == -1 {
                continue;
            }

            for k in 0..cycle.len() {
                if cycle[k] == j {
                    ans = ans.max((cycle.len() - k) as i32);
                    break;
                }
            }
        }
        ans
    }
}
