Look at what you can do now:
Build 1D (one-dimensional) and 2D prefix sum arrays
Answer any range sum query in time
Combine prefix sums with HashMaps for counting problems
Use prefix/suffix products when division isn't allowed
Apply inclusion-exclusion for 2D rectangle queries This technique is a building block. You'll use it inside DP (dynamic programming) transitions, with binary search, and in string algorithms. But the core formula never changes: .