Course_Schedule_II.cpp

class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
            vector<int> in_degree(numCourses, 0); //存储入度
			vector<unordered_set<int>> matrix(numCourses);//存储图
			vector<int> res;

			for (int i = 0; i < prerequisites.size(); i++)
			{
				matrix[prerequisites[i].second].insert(prerequisites[i].first);
			}
			//计算入度
			for (int i = 0; i < numCourses; i++)
			{
				for (auto it = matrix[i].begin(); it != matrix[i].end(); it++)
				{
					in_degree[*it]++;
				}
			}
			stack<int> zeor_degree_stack;//存储入度为0的节点
			int count = 0;//入度0的节点计数器
			for (int i = 0; i < numCourses; i++)
			{
				if (in_degree[i] == 0)
				{
					zeor_degree_stack.push(i);
					count++; //入度为0，计数加1
					res.push_back(i);
				}
			}
			//循环抽取入度为0的节点
			while (!zeor_degree_stack.empty())
			{
				int top = zeor_degree_stack.top();
				zeor_degree_stack.pop();
				for (auto it = matrix[top].begin(); it != matrix[top].end(); it++)
				{
					in_degree[*it]--;
					if (in_degree[*it] == 0)
					{
						zeor_degree_stack.push(*it);
						count++;
						res.push_back(*it);
					}
				}
			}
			if (count == numCourses)
				return res;
			else
			{
				vector<int> tmp; return tmp;
			}
    }
};