Leetcode #1972: First and Last Call On the Same Day
In this guide, we solve Leetcode #1972 First and Last Call On the Same Day 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: Calls +--------------+----------+ | Column Name | Type | +--------------+----------+ | caller_id | int | | recipient_id | int | | call_time | datetime | +--------------+----------+ (caller_id, recipient_id, call_time) is the primary key (combination of columns with unique values) for this table. Each row contains information about the time of a phone call between caller_id and recipient_id.
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 |
+--------------+----------+
| caller_id | int |
| recipient_id | int |
| call_time | datetime |
+--------------+----------+
(caller_id, recipient_id, call_time) is the primary key (combination of columns with unique values) for this table.
Each row contains information about the time of a phone call between caller_id and recipient_id.
Python Solution
import duckdb
import pandas as pd
def solution(calls: pd.DataFrame) -> pd.DataFrame:
con = duckdb.connect()
con.register("Calls", calls)
return con.execute("""with s as (
select
*
from
Calls
union
all
select
recipient_id,
caller_id,
call_time
from
Calls
),
t as (
select
caller_id user_id,
FIRST_VALUE(recipient_id) over(
partition by DATE_FORMAT(call_time, '%Y-%m-%d'),
caller_id
order by
call_time asc
) first,
FIRST_VALUE(recipient_id) over(
partition by DATE_FORMAT(call_time, '%Y-%m-%d'),
caller_id
order by
call_time desc
) last
from
s
)
select
distinct user_id
from
t
where
first = last""").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.