# 13 ms Top-sort in C++ with clear explanation

• ``````class Solution {
public:
vector<int> findOrder(int num, vector<pair<int, int>>& prereq) {
if (num == 0) return vector<int>();
// first let's count the in-degree of each point and keep record of the list in its vector
vector<vector<int>> my(num);     // each course has its own "pointed-to" vertices
vector<int> inDegree(num, 0);
vector<int> course; // to record the poped front from the queue

for (auto x : prereq){
inDegree[x.first]++;
my[x.second].push_back(x.first);
}
// Next we use topological sort to go through the graph
// we use a queue to put in the courses with 0 in-degree
// pop each of them and decrease the following courses' in-degree by 1.continue the process
std::queue<int> topsort;
for (int i = 0; i< num ; ++i){
if (inDegree[i] == 0){
topsort.push(i);
}
}
while(!topsort.empty()){
int front = topsort.front();
course.push_back(front);
for (auto x : my[front]){    // enter into the adjacent list of the current front point
if (--inDegree[x] == 0){
topsort.push(x);
}
}
topsort.pop();
}
if (course.size() == num) return course;
course.clear();
return course;
}
};
``````

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