Sliding Window — Fixed Size

Key Signals & Indicators

• Input is a linear data structure (array, string, list)
• Problem asks for a contiguous subarray or substring
• The length of the target subrange (k) is fixed/known

// C++ Fixed Sliding Window template
#include <vector>
#include <numeric>

int fixedSlidingWindow(const std::vector<int>& arr, int k) {
    if (arr.size() < k) return 0;
    
    int currentSum = 0;
    // 1. Build initial window
    for (int i = 0; i < k; ++i) {
        currentSum += arr[i];
    }
    
    int maxSum = currentSum;
    
    // 2. Slide the window over the rest of the array
    for (int i = k; i < arr.size(); ++i) {
        currentSum += arr[i] - arr[i - k]; // Add right, remove left
        maxSum = std::max(maxSum, currentSum);
    }
    
    return maxSum;
}