Sliding Window

Maintain a dynamic window over a contiguous subarray or substring to find optimal ranges in O(n) time.

When to use

• Contiguous subarray/substring
• Longest/shortest with constraint
• Fixed-size window aggregation

When NOT to use

• Non-contiguous elements needed
• Need to compare all possible subsets

Common traps

• Forgetting to shrink the window
• Off-by-one on window boundaries
• Not tracking window state correctly

Key Invariant

The window always satisfies the problem's constraint — expand right to explore, shrink left to restore validity

Problems (18)

#12Best Time to Buy and Sell Stock

#50Maximum Average Subarray I

#51Find All Anagrams in a String

#31Longest Substring Without Repeating Characters

#87Minimum Window Substring

#88Permutation in String

#89Fruit Into Baskets

#90Max Consecutive Ones III

#225Minimum Size Subarray Sum

#227Maximum Points You Can Obtain from Cards

#248Longest Repeating Character Replacement

#249Grumpy Bookstore Owner

#251Minimum Number of Flips to Make Binary String Alternating

#252Count Number of Nice Subarrays

#253Maximum Number of Vowels in a Substring of Given Length

#300Find Longest Substring With Even Vowel Counts

#119Sliding Window Maximum

#250Subarrays with K Different Integers