LeetCode 560 Subarray Sum Equals K - Why Naive Fails

Check all subarrays. For each starting index i and ending index j, compute the sum and check if it equals k.

There are $O(n^2)$ subarrays. Even with prefix sums (making each sum $O(1)$), the total is $O(n^2)$.

For $n = 2 \times 10^4$, that's $4 \times 10^8$ operations. Too slow.