package leetcode

func findOrder(numCourses int, prerequisites [][]int) []int {
	in := make([]int, numCourses)
	frees := make([][]int, numCourses)
	next := make([]int, 0, numCourses)
	for _, v := range prerequisites {
		in[v[0]]++
		frees[v[1]] = append(frees[v[1]], v[0])
	}
	for i := 0; i < numCourses; i++ {
		if in[i] == 0 {
			next = append(next, i)
		}
	}
	for i := 0; i != len(next); i++ {
		c := next[i]
		v := frees[c]
		for _, vv := range v {
			in[vv]--
			if in[vv] == 0 {
				next = append(next, vv)
			}
		}
	}
	if len(next) == numCourses {
		return next
	}
	return []int{}
}