Longest Substring Without Repeating Characters

Sliding Window · Medium · Week 2 · Phase 1

Day 12 — Longest Substring Without Repeating Characters

Coding problem

Problem	Longest Substring Without Repeating Characters
LeetCode ID(s)	LeetCode #3
Difficulty	Medium
Pattern	Sliding Window
Company tags	Meta, Netflix
Suggested time	20m

Solution outline (coding)

Focus on the Sliding Window pattern. Start by writing out a few examples by hand, then identify the invariant you must maintain (e.g., prefix sums, window bounds, visited set, heap ordering). Aim for an implementation you can explain in under a minute, including time and space complexity.

Show Python solution

# Solution template based on the main pattern for this day.
# Replace this with your final, production-ready solution as you practice.

SQL question

sessions(user_id, ts, page). BigQuery: define repeated page as same page visited twice within 10 minutes for a user; find longest interval by duration without repeated pages per user.

How to approach (SQL)

Break the prompt into steps:

Identify source tables, required joins, and filters (especially on time partitions).
Decide where you need GROUP BY vs. window functions (e.g., ROW_NUMBER, SUM() OVER, COUNT() OVER).
For BigQuery, think about partitioning and clustering to avoid unnecessary full scans.
Write the query in stages (CTEs) so each step is easy to debug and reason about. Finish by checking edge cases: nulls, late events, duplicated keys, and extreme values.