func findOrder(numCourses int, prerequisites [][]int) []int {
	adj := make([][]int, numCourses)
	in := make(map[int]int)
	for i := 0; i < numCourses; i++ {
		in[i] = 0
	}
	for i := range prerequisites {
		adj[prerequisites[i][1]] = append(adj[prerequisites[i][1]], prerequisites[i][0])
		in[prerequisites[i][0]]++
	}
	order := make([]int, 0)
	for ring := true; len(in) != 0; ring = true { // Sort by the indegree of nodes
		for k, v := range in {
			if v == 0 {
				ring = false
				order = append(order, k)
				delete(in, k)
				for i := range adj[k] {
					in[adj[k][i]]--
				}
			}
		}
		if ring {
			return []int{}
		}
	}
	return order
}