D

```
package graph
// Topological assumes that graph given is valid and that its
// possible to get a topological ordering.
// constraints are array of []int{a, b}, representing
// an edge going from a to b
func Topological(N int, constraints [][]int) []int {
dependencies := make([]int, N)
nodes := make([]int, N)
for i := range nodes {
nodes[i] = i
}
edges := make([][]bool, N)
for i := range edges {
edges[i] = make([]bool, N)
}
for _, c := range constraints {
a := c[0]
b := c[1]
dependencies[b]++
edges[a][b] = true
}
var answer []int
for s := 0; s < N; s++ {
// Only start walking from top level nodes
if dependencies[s] == 0 {
route, _ := DepthFirstSearchHelper(s, N, nodes, edges, true)
answer = append(answer, route...)
}
}
return answer
}
```