class Solution {
public:
vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
vector<int> result;
if (numCourses == 1) {
result.push_back(0);
return result;
}
vector<vector<int> > map(numCourses, vector<int>());
stack<int> reversePos;
for(int i = 0; i < prerequisites.size(); i++) {
map[prerequisites[i].second].push_back(prerequisites[i].first);
}
vector<bool> isVisited(numCourses, false);
for (int i = 0; i < numCourses; ++i) {
if (!isVisited[i]) {
vector<bool> onStack(numCourses, false);
if (hasCycle(map, i, isVisited, onStack, reversePos)) return result;
}
}
while (!reversePos.empty()) {
int index = reversePos.top();
reversePos.pop();
result.push_back(index);
}
return result;
}
bool hasCycle(vector<vector<int>> &map, int i, vector<bool> &isVisited, vector<bool> &onStack,
stack<int> &reversePos) {
isVisited[i] = true;
onStack[i] = true;
for (int k : map[i]) {
if (onStack[k])
return true;
else if (!isVisited[k])
if (hasCycle(map, k, isVisited, onStack, reversePos))
return true;
}
onStack[i] = false;
reversePos.push(i);
return false;
}
};