Sliding Window DSA All Patterns & How to recognize
🧩 Pattern 1: Fixed-Size Sliding Window
When to recognize it
Window size is given or constant
You don’t need to grow or shrink dynamically
Keywords
“subarray of size k”
“exactly k length”
“window size is k”
Template
expand right
remove left (when window size > k)
update answer
Examples
Maximum sum subarray of size k
First negative number in every window of size k
Sliding window average
TC
O(n)
🧩 Pattern 2: Variable Window – Constraint Based
When to recognize it
Window size is not fixed
You shrink only when a condition is violated
Keywords
“at most k”
“no more than”
“sum ≤ k”
“distinct ≤ k”
Key Property
✅ Window validity is monotonic
Template
expand right
while (condition violated)
shrink left
update answer
Examples
Longest substring with at most K distinct characters
Subarrays with sum ≤ K
Fruit Into Baskets
🧩 Pattern 3: Exactly K = AtMost(K) − AtMost(K−1)
When to recognize it
Problem says “exactly k”
Counting subarrays (not longest)
Keywords
“exactly k distinct”
“exactly k odd numbers”
Trick
Exactly K = AtMost(K) - AtMost(K-1)
Examples
Subarrays with K distinct integers
Nice subarrays (k odd numbers)
TC
O(n)
🧩 Pattern 4: Substring with Required Characters (Frequency Matching)
When to recognize it
Order does NOT matter
Frequency matters
Characters must be present
Keywords
“contains all characters”
“minimum window substring”
“permutation”
Key Idea
Use frequency map
Track
formed === required
Examples
Minimum Window Substring
Find All Anagrams in a String
Permutation in String
🧩 Pattern 5: Longest Window with Constraint
When to recognize it
Asked for maximum length
Window stays valid as long as constraint holds
Keywords
“longest”
“maximum length”
“replace at most k characters”
Examples
Longest Repeating Character Replacement
Longest Substring Without Repeating Characters
Trick
Don’t shrink aggressively
Track best length
🧩 Pattern 6: Forward + Backward Window (Subsequence)
When to recognize it
Order matters
Window validity is not monotonic
Subsequence, not substring
Keywords
“subsequence”
“characters must appear in order”
“minimum window subsequence”
Strategy
Forward scan → find valid window
Backward scan → minimize window
Example
- Minimum Window Subsequence
TC
O(n × m)
🧩 Pattern 7: Sliding Window + HashSet (Uniqueness)
When to recognize it
No duplicates allowed
Frequency doesn’t matter, only presence
Keywords
“without repeating”
“unique characters”
Example
- Longest Substring Without Repeating Characters
🧩 Pattern 8: Sliding Window with Prefix Trick (Advanced)
When to recognize it
Window sum doesn’t behave monotonically
Negative numbers involved
Keywords
“subarray sum equals k”
negative values present
Strategy
Prefix sum + hashmap
NOT classical sliding window
