Leetcode 207. Course Schedule

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

We can do it using graph.

Solution:

# T:O(|V|+|E|) S:O(|E|)
class Solution:
    # @param {integer} numCourses
    # @param {integer[][]} prerequisites
    # @return {boolean}
    def canFinish(self, numCourses, prerequisites):
        zero_in_degree_queue, in_degree, out_degree = collections.deque(), {}, {}
        
        for i, j in prerequisites:
            if i not in in_degree:
                in_degree[i] = sets.Set()
            if j not in out_degree:
                out_degree[j] = sets.Set()
            in_degree[i].add(j)
            out_degree[j].add(i)
        
        for i in xrange(numCourses):
            if i not in in_degree:
                zero_in_degree_queue.append(i)
        
        while zero_in_degree_queue:
            prerequisite = zero_in_degree_queue.popleft()
            
            if prerequisite in out_degree:
                for course in out_degree[prerequisite]:
                    in_degree[course].discard(prerequisite)
                    if not in_degree[course]:
                        zero_in_degree_queue.append(course)
            
                del out_degree[prerequisite]
        
        if out_degree:
            return False
        
        return True

Run Time: 108 ms