itertools.accumulate running total

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Accepted Answer

itertools.accumulate yields running totals: 1, 1+2=3, 3+3=6, 6+4=10, 10+5=15. The result includes one value per input element, starting with the first element unchanged. The last value is the sum of the entire input. This is useful for prefix sum arrays, which let you answer range-sum queries in O(1) after O(n) setup - a pattern that appears often in interview problems involving subarray sums.