When should I use sliding window?

Use it when: Contiguous subarray or substring constraint. You can move one boundary based on a monotone condition. Brute force is O(n·k) or O(n²).

What are the most common sliding window bugs?

Shrinking when you should extend first (window can miss candidates). Forgetting to update the answer after every right-move, not just after shrinks. Using < vs <= on the window condition — off-by-one on equality.

What problems use the sliding window pattern?

Representative scenarios: Pharmacy Pickup — longest window with ≤ k distinct items. Rolling Peak Sensor — max in every window of size k. Unique DNA Run — longest substring with no repeated character.

Is sliding window on the Blind 75?

Yes — Sliding Window is one of the core patterns behind the Blind 75 list. See /blind-75 for the complete mapping.

Pattern · Time O(n), space O(1) or O(k) for a frequency map.

Sliding Window — the shape behind dozens of interview problems.

Q: What is the sliding window pattern?

Sliding Window is maintain a contiguous range and slide its boundaries to avoid recomputation. Time O(n), space O(1) or O(k) for a frequency map.

Q: What problems use the sliding window pattern?

Representative scenarios: Pharmacy Pickup — longest window with ≤ k distinct items. Rolling Peak Sensor — max in every window of size k. Unique DNA Run — longest substring with no repeated character.

Q: Is sliding window on the Blind 75?

Yes — Sliding Window is one of the core patterns behind the Blind 75 list. See /blind-75 for the complete mapping.

TL;DR

Maintain a contiguous range and slide its boundaries to avoid recomputation. Time O(n), space O(1) or O(k) for a frequency map.

When to reach for Sliding Window

Contiguous subarray or substring constraint.
You can move one boundary based on a monotone condition.
Brute force is O(n·k) or O(n²).

Scenario variants (legal, original)

Pharmacy Pickup — longest window with ≤ k distinct items.
Rolling Peak Sensor — max in every window of size k.
Unique DNA Run — longest substring with no repeated character.

Sliding Window in Python

def longest_unique(s: str) -> int:
    seen = {}
    left = best = 0
    for right, ch in enumerate(s):
        if ch in seen and seen[ch] >= left:
            left = seen[ch] + 1
        seen[ch] = right
        best = max(best, right - left + 1)
    return best

See language-specific tabs at /languages/python/patterns and the SQL variant at /languages/sql/patterns.

Common bugs

Shrinking when you should extend first (window can miss candidates).
Forgetting to update the answer after every right-move, not just after shrinks.
Using < vs <= on the window condition — off-by-one on equality.

Related patterns

Two Pointers Prefix & Suffix Monotonic Stack Back to all 12

Frequently asked questions

What is the sliding window pattern?: Sliding Window is maintain a contiguous range and slide its boundaries to avoid recomputation. Time O(n), space O(1) or O(k) for a frequency map.
When should I use sliding window?: Use it when: Contiguous subarray or substring constraint. You can move one boundary based on a monotone condition. Brute force is O(n·k) or O(n²).
What are the most common sliding window bugs?: Shrinking when you should extend first (window can miss candidates). Forgetting to update the answer after every right-move, not just after shrinks. Using < vs <= on the window condition — off-by-one on equality.
What problems use the sliding window pattern?: Representative scenarios: Pharmacy Pickup — longest window with ≤ k distinct items. Rolling Peak Sensor — max in every window of size k. Unique DNA Run — longest substring with no repeated character.
Is sliding window on the Blind 75?: Yes — Sliding Window is one of the core patterns behind the Blind 75 list. See /blind-75 for the complete mapping.

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