A shared descendant is not necessarily a directed cycle

package main

import "fmt"

type frame struct{ node, next int }

func cyclic(g [][]int) (bool, error) {
	colors := make([]uint8, len(g))
	for root := range g {
		if colors[root] != 0 {
			continue
		}
		colors[root] = 1
		stack := []frame{{node: root}}
		for len(stack) > 0 {
			top := &stack[len(stack)-1]
			if top.next == len(g[top.node]) {
				colors[top.node] = 2
				stack = stack[:len(stack)-1]
				continue
			}
			child := g[top.node][top.next]
			top.next++
			if child < 0 || child >= len(g) {
				return false, fmt.Errorf("invalid edge")
			}
			if colors[child] == 1 {
				return true, nil
			}
			if colors[child] == 0 {
				colors[child] = 1
				stack = append(stack, frame{node: child})
			}
		}
	}
	return false, nil
}
func main() {
	fmt.Println(cyclic([][]int{{1, 2}, {3}, {3}, {}}))
	fmt.Println(cyclic([][]int{{1}, {2}, {0}}))
}