package q210

func findOrder(numCourses int, prerequisites [][]int) []int {
	ndeps := make([]int, numCourses)
	revDeps := make([][]int, numCourses)
	ret := make([]int, 0, numCourses)

	for i := range prerequisites {
		c, dep := prerequisites[i][0], prerequisites[i][1]
		ndeps[c]++
		revDeps[dep] = append(revDeps[dep], c)
	}

	for i := range ndeps {
		if ndeps[i] == 0 {
			ret = append(ret, i)
		}
	}
	for i := 0; i < len(ret); i++ {
		for _, c := range revDeps[ret[i]] {
			ndeps[c]--
			if ndeps[c] == 0 {
				ret = append(ret, c)
			}
		}
	}

	if len(ret) != numCourses {
		return nil
	}
	return ret
}

var _ = findOrder