二叉树的先序、中序、后序及层次遍历的非递归方式实现（Java）

public class PreInPosTraversal {   
    public static void preOrderUnRecur(Node head) {
        System.out.print("pre-order: ");
        if (head != null) {
            Stack stack = new Stack();// 辅助栈
            stack.add(head);
            while (!stack.isEmpty()) {
                head = stack.pop();
                System.out.print(head.value + " ");
                if (head.right != null) {
                    stack.push(head.right);
                }
                if (head.left != null) {
                    stack.push(head.left);
                }
            }
        }
        System.out.println();
    }

    public static void inOrderUnRecur(Node head) {
        System.out.print("in-order: ");
        if (head != null) {
            Stack stack = new Stack();
            while (!stack.isEmpty() || head != null) {
                if (head != null) {
                    stack.push(head);
                    head = head.left;
                } else {
                    head = stack.pop();
                    System.out.print(head.value + " ");
                    head = head.right;
                }
            }
        }
        System.out.println();
    }

    public static void posOrderUnRecur1(Node head) {
        System.out.print("pos-order: ");
        if (head != null) {
            Stack s1 = new Stack();//双栈法
            Stack s2 = new Stack();
            s1.push(head);
            while (!s1.isEmpty()) {
                head = s1.pop();
                s2.push(head);
                if (head.left != null) {
                    s1.push(head.left);
                }
                if (head.right != null) {
                    s1.push(head.right);
                }
            }
            while (!s2.isEmpty()) {
                System.out.print(s2.pop().value + " ");
            }
        }
        System.out.println();
    }

    public static void posOrderUnRecur2(Node h) {
        System.out.print("pos-order: ");
        if (h != null) {
            Stack stack = new Stack();
            stack.push(h);
            Node c = null;
            while (!stack.isEmpty()) {
                c = stack.peek();
                if (c.left != null && h != c.left && h != c.right) {
                    stack.push(c.left);
                } else if (c.right != null && h != c.right) {
                    stack.push(c.right);
                } else {
                    System.out.print(stack.pop().value + " ");
                    h = c;
                }
            }
        }
        System.out.println();
    }

    public static void levelTraversal(Node root){
        if(root == null) {
             return;
        }
        Queue queue = new LinkedList<>(); // 对列保存树节点
        queue.add(root);
        while (!queue.isEmpty()) {
             Node temp = queue.poll();
             System.out.print(temp.val + "->");
             if (temp.left != null) { // 添加左右子节点到对列
                  queue.add(temp.left);
             }
             if (temp.right != null) {
                  queue.add(temp.right);
             }
        }
    }
}