# Topological sort | Course schedule 2 | Leetcode #210

```There are a total of `n` courses you have to take labelled from `0` to `n - 1`.
Some courses may have `prerequisites`, for example, if `prerequisites[i] = [ai, bi]` this means you must take the course `bi` before the course `ai`.
Given the total number of courses `numCourses` and a list of the `prerequisite` pairs, return the ordering of courses you should take to finish all courses.
If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array.
Input: numCourses = 2, prerequisites = [[1,0]]
Output: [0,1]
Explanation: 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].```

This explains a very important programming interview concept which is based on graph algorithm and is known as topological sort.This is a very important concept for solving graph problems based on ordering.We can find the topological sort using the simple DFS along with a STACK.Topological sort for a graph is only possible if it is a directed acyclic graph (DAG).I have explained the entire algorithm by taking proper examples and have also shown the code implementation using a problem course schedule 2 which is from leetcode #210.I have shown the code walk through at the end of the video. CODE is present below as usual.

```class Solution {
//Graph coloring: 0->not visited...1->visited...2->visited & processed
bool detectCycle_util(vector<vector<int>>& adj,vector<int>& visited,int v)
{
if(visited[v]==1)
return true;
if(visited[v]==2)
return false;

visited[v]=1;   //Mark current as visited
for(int i=0;i<adj[v].size();++i)
if(detectCycle_util(adj,visited,adj[v][i]))
return true;

visited[v]=2;   //Mark current node as processed
return false;
}

//Cycle detection
bool detectCycle(vector<vector<int>>& adj,int n)
{
vector<int> visited(n,0);
for(int i=0;i<n;++i)
if(!visited[i])
if(detectCycle_util(adj,visited,i))
return true;
return false;
}

//Topological sort
void dfs(vector<vector<int>>& adj,int v,vector<bool>& visited,stack<int>& mystack)
{
visited[v] = true;
for(int i=0;i<adj[v].size();++i)
if(!visited[adj[v][i]])
dfs(adj,adj[v][i],visited,mystack);

mystack.push(v);
}
public:
vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites) {
int n=prerequisites.size();
vector<vector<int>> adj(numCourses);
//Make adjacecncy list
for(int i=0;i<n;++i)
adj[prerequisites[i]].push_back(prerequisites[i]);

//Detect CYCLE...If present then return empty array
vector<int> ans;
if(detectCycle(adj,numCourses))
return ans;

//Find toposort and store it in stack
stack<int> mystack;
vector<bool> visited(numCourses,false);

//Apply DFS and find topological sort
for(int i=0;i<numCourses;++i)
if(!visited[i])
dfs(adj,i,visited,mystack);

while(!mystack.empty())
{
ans.push_back(mystack.top());
mystack.pop();
}
return ans;
}
};```