不使用递归遍历二叉树

import java.util.Stack;

public class AnswerB13 {

public static void main(String[] args) {
Node tree = makeupTree();
Stack<Node> stack = new Stack<Node>();
Node currentNode = tree;
while (currentNode != null) {
System.out.println(currentNode.value);
stack.push(currentNode);
currentNode = currentNode.left;
if (currentNode == null) {
Node parent = stack.pop();
currentNode = parent.right;
if (currentNode == null) {
if (stack.isEmpty()) {
break;
}
Node parentParent = stack.pop();
currentNode = parentParent.right;
}
}
}
}

private static Node makeupTree() {
Node root = new Node(0);
Node node1 = new Node(1);
Node node2 = new Node(2);
Node node11 = new Node(11);
Node node12 = new Node(12);
Node node21 = new Node(21);
Node node22 = new Node(22);

root.left = node1;
root.right = node2;

node1.left = node11;
node1.right = node12;

node2.left = node21;
node2.right = node22;
return root;
}

public static class Node {
public Node left;
public Node right;
public int value;

public Node(int value) {
this.value = value;
}
}