Course Schedule II

Course Schedule II

There are a total of n courses you have to take labelled from 0 to n - 1.

Some courses may have prerequisites, for example, if prerequisites[i] = [ai, bi] this means you must take the course bi before the course ai.

Given the total number of courses numCourses and a list of the prerequisite pairs, return the ordering of courses you should take to finish all courses.

If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array.

Example 1:

Input: numCourses = 2, prerequisites = [[1,0]]
Output: [0,1]
Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1].

Example 2:

Input: numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]]
Output: [0,2,1,3]
Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0.
So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3].

Example 3:

Input: numCourses = 1, prerequisites = []
Output: [0]

Constraints:

• 1 <= numCourses <= 2000

• 0 <= prerequisites.length <= numCourses * (numCourses - 1)

• prerequisites[i].length == 2

• 0 <= ai, bi < numCourses

• ai != bi

• All the pairs [ai, bi] are distinct.

Solutions

🧠 Cpp

#include <memory>
#include <functional>

struct Course
{
    int id;
    bool learned = false;
    vector<Course*> depends_on;

    Course(int i):id(i), learned(false){}
};

class Solution
{
    bool found_loop(Course *start, Course *course_to_check, int len, int current_len)
    {

        if(current_len > len)
            return true;
        for(auto* course : start->depends_on)
        {   if(course == course_to_check)
                return true;
            if(found_loop(course, course_to_check, len, current_len+1))
                return true;
        }
        return false;
    }
public:
    vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites)
    {
        vector<Course*> courses;
        for(int i = 0; i < numCourses; ++i)
            courses.push_back(new Course(i));

        //copy graph from input
        Course *dependancy_tree = nullptr; 
        for(auto p : prerequisites)
            courses[p[0]]->depends_on.push_back(courses[p[1]]); 

        //function to mark learned couses in order 
        vector<int> res;
        std::function<void(int)> learn_course = [&courses, &res, &learn_course](int id)
        {
            if(courses[id]->learned)
                return;

            for(auto* course : courses[id]->depends_on)
                learn_course(course->id);

            courses[id]->learned = true;
            res.push_back(id);
        };

        //check for loops
        for(auto* c : courses)
        if(found_loop(c, c, courses.size(), 1))
            return  vector<int>{};
        for(auto* c : courses)
            learn_course(c->id);
        for(auto* c : courses)
            delete c;

        return res.size() == courses.size() ? res : vector<int>{}; 


    }
};