Tree Operations 打印出有向图中的环
2014-08-17 11:32:41         来源：he wolf

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

```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 list = new ArrayList();
ArrayList> ret = new ArrayList>();
rec(node, list, ret);
System.out.println(ret);
}

public static void rec(Node node, ArrayList list, ArrayList> ret) {
if(node.color == VertexState.Gray) {
return;
}
node.color = VertexState.Gray;
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 + "";
}
}
}
```