LeetCode Sliding Window Pattern — Complete Guide

What is the Sliding Window Pattern?

Sliding Window is a technique for finding a subarray or substring that optimizes a certain condition. Instead of checking every possible subarray (O(n²) or O(n³)), the window "slides" through the array in O(n) time.

Fixed-Size vs Dynamic-Size Windows

Type	Window Size	When to Use	Example
Fixed	Constant k	"Find max sum of k consecutive elements"	Max Average Subarray I
Dynamic	Variable	"Find longest substring with condition"	Longest Substring Without Repeating Characters

Fixed-Size Sliding Window Template

function fixedWindow(nums, k) {
  let windowSum = 0, maxSum = 0;
  
  // Build initial window
  for (let i = 0; i < k; i++) {
    windowSum += nums[i];
  }
  maxSum = windowSum;
  
  // Slide the window
  for (let i = k; i < nums.length; i++) {
    windowSum += nums[i] - nums[i - k]; // add new, remove old
    maxSum = Math.max(maxSum, windowSum);
  }
  
  return maxSum;
}

Dynamic-Size Sliding Window Template

function dynamicWindow(s) {
  let left = 0, maxLen = 0;
  const seen = new Set();
  
  for (let right = 0; right < s.length; right++) {
    // Expand window by adding s[right]
    while (seen.has(s[right])) {
      // Contract window from left until valid
      seen.delete(s[left]);
      left++;
    }
    seen.add(s[right]);
    maxLen = Math.max(maxLen, right - left + 1);
  }
  
  return maxLen;
}

Top Sliding Window LeetCode Problems

Problem	Difficulty	Window Type
3. Longest Substring Without Repeating Characters	Medium	Dynamic
76. Minimum Window Substring	Hard	Dynamic
209. Minimum Size Subarray Sum	Medium	Dynamic
239. Sliding Window Maximum	Hard	Fixed
643. Maximum Average Subarray I	Easy	Fixed
424. Longest Repeating Character Replacement	Medium	Dynamic
438. Find All Anagrams in a String	Medium	Fixed
567. Permutation in String	Medium	Fixed

How to Recognize Sliding Window Problems

Look for these keywords in the problem statement:

• "Longest/shortest substring or subarray"
• "Consecutive elements"
• "Window of size k"
• "Contains all characters"
• "Maximum sum of k elements"
• "At most K distinct characters"

How GhOst Helps with Sliding Window Problems

• Instant pattern recognition: Identifies Sliding Window from keywords in the problem
• Template selection: Chooses fixed vs dynamic based on the question
• Edge cases: Empty string, all same characters, no valid window
• Optimization hints: Suggests Hash Map vs Set vs Deque for the window