給定一個二叉樹，判斷其是否是一個有效的二叉搜尋樹。

一個二叉搜尋樹具有如下特徵：

節點的左子樹只包含小於當前節點的數。
節點的右子樹只包含大於當前節點的數。
所有左子樹和右子樹自身必須也是二叉搜尋樹。

public class IsValidBST {


    @Test
    public void isValidBSTTest1() {
        TreeNode input = new TreeNode(2);
        input.left = new TreeNode(1);
        input.right = new TreeNode(3
 
);
        Assert.assertTrue(isValidBST(input));
    }

    @Test
    public void isValidBSTTest2() {
        TreeNode input = new TreeNode(2);
        input.left = new TreeNode(1);
        input.right = new TreeNode(3);
        input.left.left = new TreeNode(4);
        Assert.assertFalse(isValidBST(input));
    }

    @Test
 

    public void isValidBSTTest3() {
        TreeNode input = new TreeNode(10);
        input.left = new TreeNode(5);
        input.right = new TreeNode(15);
        input.right.left = new TreeNode(6);
        input.right.right = new TreeNode(20);
        Assert.assertFalse(isValidBST(input));
    }

    public
 
 boolean isValidBST(TreeNode root) {
        if (root == null) return true;

        Stack<TreeNode> stack = new Stack<>();
        List<TreeNode> list = new ArrayList<>();
        while (root != null || stack.size() > 0) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }

            if (stack.size() > 0) {
                TreeNode treeNode = stack.pop();
                list.add(treeNode);
                root = treeNode.right;
            }
        }

        for (int i = 1; i < list.size(); i++) {
            if (list.get(i-1).val >= list.get(i).val) {
                return false;
            }
        }

        return true;
    }
}