Course Schedule II

def findOrder(numCourses, prerequisites):
    prereq = {c: [] for c in range(numCourses)}
    for crs, pre in prerequisites:
        prereq[crs].append(pre)
        
    output = []
    visit, cycle = set(), set()
    
    def dfs(crs):
        if crs in cycle:
            return False
        if crs in visit:
            return True
            
        cycle.add(crs)
        for pre in prereq[crs]:
            if dfs(pre) == False: return False
            
        cycle.remove(crs)
        visit.add(crs)
        output.append(crs)
        return True
        
    for c in range(numCourses):
        if dfs(c) == False:
            return []
    return output