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$ curl repovive.com/roadmaps/data-structures/arrays-prefix-sums/range-sum-building-the-array

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$ curl repovive.com/roadmaps/data-structures/arrays-prefix-sums/range-sum-building-the-array

Repovive

Range Sum - Building the Array - Arrays & Prefix Sums | Data Structures | Repovive

Repovive.

Data Structures19 sections · 729 units49h total

Open in Course

Range Sum - Building the Array

One pass construction

Building the prefix array takes one pass through the input:

function buildPrefixSum(nums)
    n := length of nums
    prefix := array of size (n + 1)
    prefix[0] := 0
    for i from 0 to n - 1
        prefix[i + 1] := prefix[i] + nums[i]
    return prefix

Time: $O(n)$ . Space: $O(n)$ .

Notice how each prefix value builds on the previous one. You're not recalculating sums from scratch each time.