一、二叉树-中序遍历：左>头>右

1. 递归实现

    //中序  左中头
    public static void in(Node head) {
        if (head == null) return;
        in(head.left);
        System.out.print(head.value + ", ");
        in(head.rigth);
    }

2. 非递归实现

使用栈集合存储节点数据，完成非递归实现

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

        }
    }