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Problems > Sum over All Subarrays > Editorial

Sum over All Subarrays — Solution & Editorial

Back to the Problem

Count each element's contribution: ai (1-based i) appears in i · (N−i+1) subarrays. Sum ai times that count — no enumeration needed.

Complexity: O(N)

Watch out: The total reaches ~1018; 64-bit integers are required (Python is safe, C++ needs long long).