98. 验证二叉搜索树

    public boolean isValidBST(TreeNode root) {
        return helper(root, null, null);
    }
    private boolean helper(TreeNode root, Integer lower, Integer upper) {
        if (root == null) {
            return true;
        }
        int val = root.val;
        if (lower != null && val <= lower) {
            return false;
        }
        if (upper != null && val >= upper) {
            return false;
        }
        if (!helper(root.right, val, upper)) {
            return false;
        }
        if (!helper(root.left, lower, val)) {
            return false;
        }
        return true;
    }


    public boolean isValidBST2(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        double inorder = -Double.MAX_VALUE;
        while(!stack.isEmpty() || root != null) {
            while(root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if (root.val <= inorder) {
                return false;
            }
            inorder = root.val;
            root = root.right;
        }
        return true;
    }