Second degree follower

# Table: Follow
# 
# +-------------+---------+
# | Column Name | Type    |
# +-------------+---------+
# | followee    | varchar |
# | follower    | varchar |
# +-------------+---------+
# (followee, follower) is the primary key (combination of columns with unique values) for this table.
# Each row of this table indicates that the user follower follows the user followee on a social network.
# There will not be a user following themself.
# 
#  
# 
# A second-degree follower is a user who:
# 
# follows at least one user, and
# 
# is followed by at least one user.
# 
# Write a solution to report the second-degree users and the number of their followers.
# 
# Return the result table ordered by follower in alphabetical order.
# 
# The result format is in the following example.
#
# Example 1:
# Input:
# Follow table:
# +----------+----------+
# | followee | follower |
# +----------+----------+
# | Alice    | Bob      |
# | Bob      | Cena     |
# | Bob      | Donald   |
# | Donald   | Edward   |
# +----------+----------+
# Output:
# +----------+-----+
# | follower | num |
# +----------+-----+
# | Bob      | 2   |
# | Donald   | 1   |
# +----------+-----+
# Explanation:
# User Bob has 2 followers. Bob is a second-degree follower because he follows Alice, so we include him in the result table.
# User Donald has 1 follower. Donald is a second-degree follower because he follows Bob, so we include him in the result table.
# User Alice has 1 follower. Alice is not a second-degree follower because she does not follow anyone, so we don not include her in the result table.

import pandas as pd

def second_degree_follower(follow: pd.DataFrame) -> pd.DataFrame:
  follows_someone = set(follow['follower'].unique())
  is_followed = follow[follow['followee'].isin(follows_someone)]
  result = is_followed.groupby('followee')['follower'].nunique().reset_index()
  result.columns = ['follower', 'num']
  return result.sort_values('follower')