Leetcode #2153: The Number of Passengers in Each Bus II
In this guide, we solve Leetcode #2153 The Number of Passengers in Each Bus II 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 | | capacity | int | +--------------+------+ bus_id contains unique values. Each row of this table contains information about the arrival time of a bus at the LeetCode station and its capacity (the number of empty seats it has).
Quick Facts
- Difficulty: Hard
- 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 |
| capacity | int |
+--------------+------+
bus_id contains unique values.
Each row of this table contains information about the arrival time of a bus at the LeetCode station and its capacity (the number of empty seats it has).
No two buses will arrive at the same time and all bus capacities will be positive integers.
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("""WITH
T AS (
SELECT
*,
SUM(cnt) OVER (ORDER BY dt, bus_id) AS cur,
IF(@t > 0, @t := cnt, @t := @t + cnt) AS cur_sum
FROM
(
SELECT bus_id, arrival_time AS dt, capacity AS cnt FROM Buses
UNION ALL
SELECT -1, arrival_time AS dt, -1 FROM Passengers
) AS a JOIN (SELECT @t := 0 x) AS b
)
SELECT
bus_id,
IF(cur_sum > 0, cnt - cur_sum, cnt) AS passengers_cnt
FROM T
WHERE bus_id > 0
ORDER BY bus_id;""").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.