Leetcode #2142: The Number of Passengers in Each Bus I
In this guide, we solve Leetcode #2142 The Number of Passengers in Each Bus I in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Table: Buses +--------------+------+ | Column Name | Type | +--------------+------+ | bus_id | int | | arrival_time | int | +--------------+------+ bus_id is the column with unique values for this table. Each row of this table contains information about the arrival time of a bus at the LeetCode station.
Quick Facts
- Difficulty: Medium
- Premium: Yes
- Tags: Database
Intuition
The task is relational in nature, which maps cleanly to DataFrame operations in Python.
By treating tables as DataFrames, joins and group-bys become concise and readable.
Approach
Load the inputs as DataFrames and apply the appropriate merge, filter, or group-by.
Select or rename the columns to match the required output.
Steps:
- Load inputs as DataFrames.
- Apply merge/groupby/filter operations.
- Select the output columns.
Example
+--------------+------+
| Column Name | Type |
+--------------+------+
| bus_id | int |
| arrival_time | int |
+--------------+------+
bus_id is the column with unique values for this table.
Each row of this table contains information about the arrival time of a bus at the LeetCode station.
No two buses will arrive at the same time.
Python Solution
import duckdb
import pandas as pd
def solution(buses: pd.DataFrame, passengers: pd.DataFrame) -> pd.DataFrame:
con = duckdb.connect()
con.register("Buses", buses)
con.register("Passengers", passengers)
return con.execute("""SELECT
bus_id,
COUNT(passenger_id) - LAG(COUNT(passenger_id), 1, 0) OVER (
ORDER BY b.arrival_time
) AS passengers_cnt
FROM
Buses AS b
LEFT JOIN Passengers AS p ON p.arrival_time <= b.arrival_time
GROUP BY 1
ORDER BY 1;""").df()
Complexity
The time complexity is O(n log n) (typical). The space complexity is O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.