def countNodes(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        '''
        if the complete binary tree has all the nodes then it's leftmost node height(lheight) will be same as right most node height (rheight)
        if lheight and rheight are same => return 2^height-1 (because that are number of nodes in a whole complete BT)
        o/w count(left_sub_tree) and count(right_sub_tree) recursively
        return LST_count + RST_count+1
        '''
        def count(root):
            if root is None:
                return 0
            cur_left, cur_right = root, root
            lheight, rheight = 0, 0
            while cur_left:
                cur_left=cur_left.left
                lheight+=1
            while cur_right:
                cur_right=cur_right.right
                rheight+=1
            if lheight==rheight:
                return 2**lheight-1
            return count(root.left)+count(root.right)+1

        return count(root)