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Precompute the Running Total
Spot the range-sum questions where one precomputed table answers every query.
Multiple choiceTap - 2
Running Sum of an Array
Build the prefix table in one pass: each entry is the previous total plus the current number.
Complete codeMultiple choiceTrace - 3
Range Sum in O(1)
Turn every range-sum query into two lookups: P[j + 1] minus P[i].
Complete codeDebugMultiple choiceTap - 4
Find Pivot Index
One running left sum plus the total gives you the right sum for free — no second array, no rescans.
Complete codeMultiple choiceTrace - 5
Product of Array Except Self
Split each answer into a left product and a right product — two sweeps, no division.
Complete codeDebugMultiple choiceTrace - 6
Contiguous Array
Turn 'equal 0s and 1s' into 'same running sum twice' — a hashmap of first-seen sums does the rest.
Complete codeDebugMultiple choiceTrace - 7
Prefix Sum CheckpointCheckpoint
No new concepts — combine the island's rules: build P once with P[0] = 0, answer any range with P[j+1] - P[i], and balance a pivot against one precomputed total.
DebugCode orderFill blankMultiple choiceTapTrace - 8
Prefix Sum ReviewReview
Mixed recap of the island: the running build, O(1) range queries, pivot balance, prefix-times-suffix products, and the prefix + hashmap trick.
Complete codeDebugMultiple choiceTapTrace - 9
Boss: Subarray Sum Equals KBoss
The island's finale: a subarray sums to k exactly when currentPrefix - k has been seen before — so count every prefix in a hashmap seeded with {0: 1}.
Complete codeCode orderMultiple choiceTrace
Array Patterns is part of AlgoFox Pro
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