gistfile1.txt

    public boolean isValidBST(TreeNode root)
    {
        Integer[] prev = new Integer[1];
        return isValidBST(root, prev);
    }

    public boolean isValidBST(TreeNode root, Integer[] prev)
    {
        boolean left = true;
        if (root.left != null)
        {
            left = isValidBST(root.left, prev);
        }
        
        if (prev[0] != null && prev[0] >= root.val)
        {
            return false;
        }

        prev[0] = root.val;

        boolean right = true;
        if (root.right != null)
        {
            right = isValidBST(root.right, prev);
        }
        
        return left && right;
    }