[Python/Pandas] 리트코드 1729. Find Followers Count

문제

Table: Followers
+-------------+------+
| Column Name | Type |
+-------------+------+
| user_id     | int  |
| follower_id | int  |
+-------------+------+
(user_id, follower_id) is the primary key (combination of columns with unique values) for this table.
This table contains the IDs of a user and a follower in a social media app where the follower follows the user.

 Write a solution that will, for each user, return the number of followers.

Return the result table ordered by user_id in ascending order.

 

https://leetcode.com/problems/find-followers-count/description/?lang=pythondata

예시

Input: 
Followers table:
+---------+-------------+
| user_id | follower_id |
+---------+-------------+
| 0       | 1           |
| 1       | 0           |
| 2       | 0           |
| 2       | 1           |
+---------+-------------+
Output: 
+---------+----------------+
| user_id | followers_count|
+---------+----------------+
| 0       | 1              |
| 1       | 1              |
| 2       | 2              |
+---------+----------------+
Explanation: 
The followers of 0 are {1}
The followers of 1 are {0}
The followers of 2 are {0,1}

문제 풀이

import pandas as pd

def count_followers(followers: pd.DataFrame) -> pd.DataFrame:
    # user_id 열 기준으로 그룹, 'follower_id' 고유값 개수 계산한 새 열 'followers_count' 반환
    grouped = followers.groupby('user_id', as_index=False).agg(followers_count=('follower_id', 'nunique'))
    # 'user_id' 열을 기준으로 오름차순 정렬
    result = grouped.sort_values('user_id')
    return result

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파이썬을 독학하시는 분들에게 도움이 되길 바라며,

혹 더 좋은 방법이 있거나 오류가 있다면 편하게 말씀 부탁드립니다.