When to Use

Look for these signals in the problem:

• "Sum of subarray/range" queries: Precompute prefix sums for $O(1)$ range sum queries.
• "Subarray sum equals k": Use hash map to track prefix sums seen; find complements.
• "Count subarrays" with sum property: Prefix sum + hash map counts subarrays efficiently.
• "2D range sum" queries: Extend prefix sum to 2D with inclusion-exclusion.
• "Product except self": Prefix and suffix products avoid division issues.