package q207

func canFinish(numCourses int, prerequisites [][]int) bool {
	nDeps := make([]int, numCourses)
	revDeps := make([][]int, numCourses)

	for i := range prerequisites {
		course, prereq := prerequisites[i][0], prerequisites[i][1]
		nDeps[course]++
		revDeps[prereq] = append(revDeps[prereq], course)
	}

	queue := []int{}
	for i := range numCourses {
		if nDeps[i] == 0 && len(revDeps) > 0 {
			queue = append(queue, i)
		}
	}
	for ; len(queue) > 0; queue = queue[1:] {
		course := queue[0]
		for _, r := range revDeps[course] {
			nDeps[r]--
			if nDeps[r] == 0 {
				queue = append(queue, r)
			}
		}
	}

	for _, n := range nDeps {
		if n > 0 {
			return false
		}
	}
	return true
}

var _ = canFinish