Challenge: Off-by-One Errors

The formula $prefix[r+1] - prefix[l]$ uses 1-indexed prefixes. If your array is 0-indexed, be careful. Common mistake: using $prefix[r] - prefix[l-1]$ when $l = 0$.

This accesses $prefix[-1]$. Handle the edge case or use 1-indexed prefixes. Test your implementation: sum of $arr[0..0]$ should be $arr[0]$. Sum of $arr[0..n-1]$ should be total sum. If these fail, check your indices. I recommend 1-indexed prefixes with $prefix[0] = 0$. Then $sum(l, r) = prefix[r+1] - prefix[l]$ works for all valid ranges.