Find cities in each state

# Table: cities
# 
# +-------------+---------+
# | Column Name | Type    |
# +-------------+---------+
# | state       | varchar |
# | city        | varchar |
# +-------------+---------+
# (state, city) is the primary key (combination of columns with unique values) for this table.
# Each row of this table contains the state name and the city name within that state.
# 
# Write a solution to find all the cities in each state and combine them into a single comma-separated string.
# 
# Return the result table ordered by state and city in ascending order.
# 
# The result format is in the following example.
#
# Example 1:
# Input:
# cities table:
# +-------------+---------------+
# | state       | city          |
# +-------------+---------------+
# | California  | Los Angeles   |
# | California  | San Francisco |
# | California  | San Diego     |
# | Texas       | Houston       |
# | Texas       | Austin        |
# | Texas       | Dallas        |
# | New York    | New York City |
# | New York    | Buffalo       |
# | New York    | Rochester     |
# +-------------+---------------+
# Output:
# +-------------+---------------------------------------+
# | state       | cities                                |
# +-------------+---------------------------------------+
# | California  | Los Angeles, San Diego, San Francisco |
# | New York    | Buffalo, New York City, Rochester     |
# | Texas       | Austin, Dallas, Houston               |
# +-------------+---------------------------------------+
# Explanation:
# California: All cities ("Los Angeles", "San Diego", "San Francisco") are listed in a comma-separated string.
# New York: All cities ("Buffalo", "New York City", "Rochester") are listed in a comma-separated string.
# Texas: All cities ("Austin", "Dallas", "Houston") are listed in a comma-separated string.
# Note: The output table is ordered by the state name in ascending order.

import pandas as pd

def find_cities(cities: pd.DataFrame) -> pd.DataFrame:
  cities = cities.sort_values(['state', 'city'])
  result = cities.groupby('state')['city'].apply(lambda x: ', '.join(x)).reset_index()
  result.columns = ['state', 'cities']
  return result.sort_values('state').reset_index(drop=True)