題目：

You are given a binary tree with unique integer values on each node. However, the child pointers on each node may point to any other node in the tree including itself, introducing cycles into the binary tree. A cycle is defined when you can traverse back to the same node by following its descendants. Write a function that takes in the root node of the tree and prints out the cycles, if any, in the binary tree. The only operations available on each node are node.left (returns another Node or null), node.right, and node.value (returns the integer value of the node). Pseudocode is fine.

即找出有向圖中的環（loop或者cycle），而且所有打印出來。

Example: http://i.imgur.com/7S5fZe5.png

cycles: [1, 2, 4], [5], [3,6]

解答：

import java.util.ArrayList;

public class Graph {

    enum VertexState {
        White, Gray, Black
    }
    
    public static void main(String[] args) {
        Node node = new Node(0);
        node.color = VertexState.White;
        Node left = new Node(1);
        left.color = VertexState.White;
        node.left = left;
        Node right = new Node(2);
        right.color = VertexState.White;
        Node rightright = new Node(3);       
        node.right = right;
        left.left = node;
        right.right = rightright;
        rightright.right = node;
        
        ArrayList<Node> list = new ArrayList<Node>();
        ArrayList<ArrayList<Node>> ret = new ArrayList<ArrayList<Node>>();
        rec(node, list, ret);
        System.out.println(ret);
    }

    public static void rec(Node node, ArrayList<Node> list, ArrayList<ArrayList<Node>> ret) {
        if(node.color == VertexState.Gray) {
            ret.add(new ArrayList<Node>(list));
            return;
        }
        node.color = VertexState.Gray;
        list.add(node);
        if(node.left != null && node.left.color != VertexState.Black) {
            rec(node.left, list, ret);
        }
        if(node.right != null && node.right.color != VertexState.Black) {
            rec(node.right, list, ret);
        }
        list.remove(list.size()-1);
        node.color = VertexState.Black;
    }
    
    public static class Node {
        int val;
        Node left;
        Node right;
        VertexState  color;
        public Node(int val_) {
            val = val_;
        }
        @Override
        public String toString() {
            return this.val + "";
        }
    }
}

Tree Operations 打印出有向圖中的環