<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 {
public:
//   void toplogy(int s, vector<int> &vis ,vector<vector<int>> &g, stack<int> &st){
//       vis[s]=1;
//       for(auto child:g[s]){
//         if(!vis[child]){
//         toplogy(child,vis,g,st);
//         }
//       }
//       st.push(s);
//     }

// bool dfs(int s,vector<int>&vis,vector<int> &ref,int &length,vector<vector<int>>g){
//        vis[s]=1;
//        ref[s]=1;
//        bool ans = false;
//        for(auto child:g[s]){
//  if(!vis[child]){
//   if(dfs(child,vis,ref,length+1,g)==true){
//      ans|=true;
//   }
//  }
//   else if(ref[child]){
//     return true;
//    }
//        }
//         ref[s]=0;
//         length=0;
//        return ans;
// }

  int cycle(int s,vector<vector<int>> &g,vector<int>&vis,set<int>&st){
     vis[s]=1;
       st.insert(s);
      for(auto child:g[s]){
       if(st.find(child)!=st.end()) return child;
       if(!vis[child]){
  int x  = cycle(child,g,vis,st);
  if(x!=-1){
    return x;
  }
       }
       
      }
      st.erase(s);
      return -1;
  }
    int longestCycle(vector<int>& edges) {
        int n = edges.size();
          vector<vector<int>>g(n);
           for(int i=0;i<n;i++){
             if(edges[i]==-1) continue;
             g[edges[i]].push_back(i);

           }
           vector<int>vis(n,0);
           //vector<int>ref(n,0);
           int length =-1;
          
           set<int>st;
            for(int i=0;i<n;i++){
                if(!vis[i]){
                    int x = cycle(i,g,vis,st);
                  //  cout<<x<<" ";
                    if(x!=-1){
                        int temp = i;
                        int l =1;
                        while(edges[temp]!=i){
                         l++;
                         temp = edges[temp];
                        }
                        length = max(length,l);
                    }
                }
            }
            
   return length;
    }
};
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