Hey great article, it’s so helpful!

But one thing I wanted to point out as I was actually following your examples step by step:

int max_sum = 0, window_sum = 0;
/* calculate sum of 1st window */
for (int i = 0; i < k; i++)  window_sum += arr[i];
/* slide window from start to end in array. */
 
for (int i = k; i < n; i++){ 
    window_sum += arr[i] - arr[i-k];    // saving re-computation
    max_sum = max(max_sum, window_sum);
}

I don’t think this gives the correct max_sum for arr = [700, 800, 900, 1, 2] and k = 3, because your very first window_sum 700+800+900 is never actually compared against max_sum.

Before we have a chance to compare against max_sum and set max_sum equal to that window_sum, you modify it in the first iteration of your second for-loop, I think.

Please let me know if I’m wrong!