<p>There are a total of <code>numCourses</code> courses you have to take, labeled from <code>0</code> to <code>numCourses - 1</code>. You are given an array <code>prerequisites</code> where <code>prerequisites[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> indicates that you <strong>must</strong> take course <code>b<sub>i</sub></code> first if you want to take course <code>a<sub>i</sub></code>.</p>

<ul>
	<li>For example, the pair <code>[0, 1]</code>, indicates that to take course <code>0</code> you have to first take course <code>1</code>.</li>
</ul>

<p>Return <em>the ordering of courses you should take to finish all courses</em>. If there are many valid answers, return <strong>any</strong> of them. If it is impossible to finish all courses, return <strong>an empty array</strong>.</p>

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

<pre>
<strong>Input:</strong> numCourses = 2, prerequisites = [[1,0]]
<strong>Output:</strong> [0,1]
<strong>Explanation:</strong> There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1].
</pre>

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

<pre>
<strong>Input:</strong> numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]]
<strong>Output:</strong> [0,2,1,3]
<strong>Explanation:</strong> There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0.
So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3].
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> numCourses = 1, prerequisites = []
<strong>Output:</strong> [0]
</pre>

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

<ul>
	<li><code>1 &lt;= numCourses &lt;= 2000</code></li>
	<li><code>0 &lt;= prerequisites.length &lt;= numCourses * (numCourses - 1)</code></li>
	<li><code>prerequisites[i].length == 2</code></li>
	<li><code>0 &lt;= a<sub>i</sub>, b<sub>i</sub> &lt; numCourses</code></li>
	<li><code>a<sub>i</sub> != b<sub>i</sub></code></li>
	<li>All the pairs <code>[a<sub>i</sub>, b<sub>i</sub>]</code> are <strong>distinct</strong>.</li>
</ul>

class Solution {
public:
    void dfs(int i,vector<int>adj[],vector<bool>&visited,vector<int>&final)
    {
        if(visited[i])
            return;
        visited[i]=true;
        for(int j:adj[i])
        {
            dfs(j,adj,visited,final);
        }
        final.push_back(i);
    }
    bool isCycle(int num,vector<int>adj[],vector<int>&topsort)
    {
        //vector<int>topsort;
        vector<int>indexpos(num,0);
        vector<bool>visited(num,false);
        for(int i=0;i<num;i++)
        {
            if(!visited[i])
            {
                dfs(i,adj,visited,topsort);
            }
        }
        reverse(topsort.begin(),topsort.end());
        for(int i=0;i<num;i++)
        {
            cout<<topsort[i]<<endl;
            indexpos[topsort[i]]=i;
        }
        for(int i=0;i<num;i++)
        {
            for(int j : adj[i])
            {
                cout<<j<<endl;
                cout<<i<<endl;
                if(indexpos[i]>=indexpos[j])
                {
                    return true;
                }
            }
        }
        return false;
    }
    vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites) {
        vector<int>adj[numCourses];
        for(auto it:prerequisites)
        {
            adj[it[1]].push_back(it[0]);
        }
        vector<int>topsort;
        return !isCycle(numCourses,adj,topsort) ? topsort: vector<int>();
        // vector<int>indegree(numCourses,0);
        // for(int i=0;i<numCourses;i++)
        // {
        //     for(auto j : adj[i])
        //     {
        //         indegree[j]++;
        //     }
        // }
        // queue<int>q;
       
        // for(int i=0;i<numCourses;i++)
        // {
        //     if(indegree[i]==0)
        //         q.push(i);
        // }
        // while(!q.empty())
        // {
        //     int y=q.front();
        //     topsort.push_back(y);
        //     q.pop();
        //     for(auto i : adj[y])
        //     {
        //         indegree[i]--;
        //         if(indegree[i]==0)
        //             q.push(i);
        //     }
        // }
        // return topsort.size()==numCourses ? topsort: vector<int>();
        
    }
};
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