之字形遍历二叉树——你为何这么浪

    public void loop(TreeNode root) {
        if (root == null) return;

        ArrayList<TreeNode> res = new ArrayList<>();

        Stack<TreeNode> stack1 = new Stack<>();
        Stack<TreeNode> stack2 = new Stack<>();
        boolean left2Right = true;

        stack1.push(root);

        while (!stack1.isEmpty() || !stack2.isEmpty()) {
            // 从左向右
            if (left2Right) {

                while (!stack1.isEmpty()) {
                    TreeNode node = stack1.pop();
                    res.add(node);
                    if (node.left != null) {
                        stack2.push(node.left);
                    }
                    if (node.right != null) {
                        stack2.push(node.right);
                    }
                }
            // 从右向左
            } else {

                while (!stack2.isEmpty()) {
                    TreeNode node = stack2.pop();
                    res.add(node);
                    if (node.right != null) {
                        stack1.push(node.right);
                    }
                    if (node.left != null) {
                        stack1.push(node.left);
                    }
                }

            }
            left2Right = !left2Right;
        }

        for (TreeNode node : res) {
            System.out.println("" + node.value);
        }
    }