LeetCode 210. Course Schedule II (topological sorting-determining whether a ring exists in a directed graph)

LeetCode 210. Course Schedule II (topological sorting-determining whether a ring exists in a directed graph)

Similar to LeetCode 207. Course Schedule (topological sorting-finding whether a ring exists in a directed graph.

 

Note that in for (auto p: prerequistites), the possible parallel edge or self-ring in the input is determined.

 

Code:

 

class Solution {public:    vector
 
   findOrder(int numCourses, vector
  
   >& prerequisites)     {    // [0, {1, 2, 3}], means after finishing #0, you might be able to take #1, #2, #3    // That is, you must finish #0, before trying to take #1, #2, #3    map
   
    > course_chain;    vector
    
      in_degree(numCourses, 0);    queue
     
       q;    vector
      
        ret; for (auto p: prerequisites) { // self-loop, return empty vector. if (p.first == p.second) { return vector
       
        (); } // no duplicate edges input if (find(course_chain[p.second].begin(), course_chain[p.second].end(), p.first) == course_chain[p.second].end()) { course_chain[p.second].push_back(p.first); ++ in_degree[p.first]; } } for (size_t i = 0; i < numCourses; ++ i) { if (in_degree[i] == 0) { q.push(i); } } for (; !q.empty(); q.pop()) { int pre_course = q.front(); ret.push_back(pre_course); for (auto it = course_chain[pre_course].begin(); it != course_chain[pre_course].end(); ++ it) { -- in_degree[*it]; if (in_degree[*it] == 0) { q.push(*it); } } } return ret.size()==numCourses? ret: vector
        
         (); }};