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Course Schedule II

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Description

https://leetcode.com/problems/course-schedule-ii/description/

Solution

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class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<int> degree(numCourses, 0);
        vector<vector<int>> graph(numCourses, vector<int>() );
        vector<int> ret;
        for (auto& pa : prerequisites) {
            degree[pa.first] += 1;
            graph[pa.second].push_back(pa.first);
        }
        queue<int> waitingList;
        for (int i = 0; i < numCourses; ++i) {
            if (degree[i] == 0){
                waitingList.push(i);
                ret.push_back(i);
            }
        }
        while (!waitingList.empty()) {
            int item = waitingList.front();
            waitingList.pop();
            vector<int>& lis = graph[item];
            for (int item : lis) {
                --degree[item];
                if (degree[item] == 0) {
                    waitingList.push(item);
                    ret.push_back(item);
                }
            }
        }
        
        for (int i = 0; i < numCourses; ++i)
            if (degree[i] != 0) return vector<int>();
        
        return ret;
    }
};