Fixed-Size vs Variable-Size Sliding Window

The difference between fixed-size and variable-size sliding window is whether the window has a constant size or changes dynamically. In a fixed-size window, the problem tells you the exact size k, like find the maximum sum of any five consecutive elements. You compute the first window of five elements, then slide it across the array by removing the leftmost element and adding the next element on the right. The window always has exactly five elements. The loop structure is simple, one for loop starting at index k, and in each iteration you do windowSum equals windowSum minus arr at i minus k plus arr at i. It's O of n time. In a variable-size window, the problem gives you a condition, not a size. For example, find the longest substring with at most two distinct characters. You don't know the answer length in advance. You use two pointers, left and right. The right pointer expands to add elements to the window. When the condition breaks, like you now have three distinct characters, you shrink from the left until the condition is valid again. The window size changes throughout the process. The loop structure is a for loop that moves right, and inside it there's a while loop that moves left when needed. It's still O of n because each element is processed at most twice. A fixed-size update cannot solve a variable-size condition unless the valid window length is known in advance. If you see the words longest or shortest with a condition, or at most K, that's variable-size. If you see size k or exactly k elements, that's fixed-size.