<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://assets.leetcode.com/uploads/2022/06/08/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://assets.leetcode.com/uploads/2022/06/07/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>

class Solution:
    def longestCycle(self, edges: List[int]) -> int:
        n = len(edges)
        graph = {}
        indegrees = {i:0  for i in range(n)}
        
        for source, destination in enumerate(edges):
            if destination == -1:
                continue
            graph[source] = destination
            indegrees[destination] += 1

        # print(graph, indegrees)

        stack = []
        for node in indegrees:
            if indegrees[node] == 0:
                stack.append(node)
        
        while stack:
            cur_node = stack.pop()

            if cur_node in graph:
                next_node = graph[cur_node]

                indegrees[next_node] -= 1
                if indegrees[next_node] == 0:
                    stack.append(next_node)

            del indegrees[cur_node]
        
        # no cycle detected
        if len(indegrees) == 0:
            return -1

        # print(indegrees)

        # iterate through and identify cycles
        max_cycle_length = 0
        visited = set()
        stack = []
        
        while indegrees:
            start = list(indegrees.keys())[0]
            current = graph[start]
            del indegrees[current]
            cur_cycle_length = 1
            while current != start:
                cur_cycle_length += 1
                current = graph[current]
                del indegrees[current]
            max_cycle_length = max(max_cycle_length, cur_cycle_length)
        
        return max_cycle_length


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