For the input 1,[ ] the expected output shown is "Special judge: No expected output available." why is it so ?

• class Solution {
public:
vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites)
{
map<int,unordered_set<int> > graph;
for(int i=0;i<prerequisites.size();i++)
{
int x = prerequisites[i].first;
int y = prerequisites[i].second;
graph[x].insert(y);
}
if(graph.size() == 0 || prerequisites.size() == 0)
{
vector<int> temp;
return temp;
}
vector<int> stk;
vector<int> visited;
for(int i=0;i<numCourses;i++)
{
visited.push_back(0);
stk.push_back(0);
}
for(int i=0;i<numCourses;i++)
{
if(visited[i] == 0)
{
visited[i] = 1;
stk[i] = 1;
if(detect_cycle(graph,i,visited,stk))
{
vector<int> temp;
return temp;
}
}
}
return order;
}
bool detect_cycle(map<int,unordered_set<int> >&graph,int i,vector<int>& visited,vector<int>& stk)
{
unordered_set<int>::iterator it=graph[i].begin();
for(;it != graph[i].end();it++)
{
if(stk[*it] == 1)
{
return true;
}
else
{
if(visited[*it] == 0)
{
visited[*it] = 1;
stk[*it] = 1;
if(detect_cycle(graph,*it,visited,stk))
{
return true;
}
}
}
}
stk[i] = 0;
order.push_back(i);
return 0;
}
private:
vector<int> order;
};

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