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Sliding Window
Medium

Overview (Fixed Length)

Master the fixed-size sliding window pattern: slide one element out, slide one element in.
15 min
Solve on LeetCode
What is a Fixed Length Window?

In a Fixed Length sliding window, the size of the window k never changes. Visualizing it is simple: imagine a rigid frame of size k sliding across your array one step at a time.

The Golden Rule:
At each step, we do exactly two things in O(1) time:

  1. Remove the element leaving the window (the one at i - k).
  2. Add the element entering the window (the one at i).

This "Add-Remove" strategy avoids re-calculating the window content from scratch, turning an O(N*K) brute force algorithm into an O(N) linear solution.

Core Problem: Maximum Sum Subarray of Size K

The classic problem to learn this pattern:

Given an array of integers nums and an integer k, find the maximum sum of any contiguous subarray of size k.

Example:

  • Input: nums = [2, 1, 5, 1, 3, 2], k = 3
  • Output: 9
  • Explanation: Subarray [5, 1, 3] has the largest sum.
Algorithm Strategy
  1. Initialize: Calculate the sum of the first k elements. Set this as your initial current_sum and max_sum.
  2. Slide: Iterate from index k to the end of the array.
  3. Update: For each new element at index i:
    • Add nums[i] to current_sum.
    • Subtract nums[i - k] (the element falling out) from current_sum.
    • Update max_sum = max(max_sum, current_sum).
  4. Return: max_sum.
Visual Walkthrough
Step 1 / 1

Initializing...

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