LeetCode 523 Continuous Subarray Sum - Why Naive Fails

Check all subarrays of length at least 2. For each, compute the sum and check if it's divisible by k.

There are $O(n^2)$ subarrays. Even with prefix sums making each sum $O(1)$, that's still $O(n^2)$.

For $n = 10^5$, that's $10^{10}$ operations. Too slow.