How does Interview Prep handle subarray sum?

Interview Prep covers this with 2 copy-ready snippets on this page. The "Prefix Sum" snippet in Interview Prep uses `Build a cumulative sum array so any range sum can be computed in O(1).`.

Which code does the Interview Prep example use?

The "Prefix Sum" snippet uses `Build a cumulative sum array so any range sum can be computed in O(1).`, from the Arrays & Strings section of the Interview Prep cheat sheet.

What other Interview Prep snippets are shown for "subarray sum"?

Besides "Prefix Sum", this page also shows "Subarray Sum Equals K".

Subarray sum

2 snippets in Interview Prep

DSAInterview Prep

Prefix Sum

DSA · Arrays & Strings

Syntax

Build a cumulative sum array so any range sum can be computed in O(1).
prefix[i] = sum of arr[0..i-1]
Range sum [l..r] = prefix[r+1] - prefix[l]

Example

// JavaScript
function buildPrefix(arr) {
  const prefix = [0];
  for (const val of arr) {
    prefix.push(prefix[prefix.length - 1] + val);
  }
  return prefix;
}
function rangeSum(prefix, left, right) {
  return prefix[right + 1] - prefix[left];
}

# Python
def build_prefix(arr):
    prefix = [0]
    for val in arr:
        prefix.append(prefix[-1] + val)
    return prefix

def range_sum(prefix, left, right):
    return prefix[right + 1] - prefix[left]

Output

arr=[3,1,4,1,5] → prefix=[0,3,4,8,9,14]
rangeSum(1,3) = prefix[4]-prefix[1] = 9-3 = 6

Note Build: O(n) time, O(n) space. Query: O(1). Extremely useful when you need many range sum queries. Variant: prefix XOR for range XOR problems. For 2D grids, use 2D prefix sums with inclusion-exclusion.

Subarray Sum Equals K

DSA · Hash Maps & Sets

Syntax

Use prefix sum + hash map.
At each index, check if (currentPrefix - k) was seen before.
Store prefix sum frequencies in the map.

Example

// JavaScript
function subarraySum(nums, k) {
  const prefixCount = new Map([[0, 1]]);
  let prefix = 0, count = 0;
  for (const num of nums) {
    prefix += num;
    if (prefixCount.has(prefix - k)) {
      count += prefixCount.get(prefix - k);
    }
    prefixCount.set(prefix, (prefixCount.get(prefix) || 0) + 1);
  }
  return count;
}

# Python
def subarray_sum(nums, k):
    prefix_count = {0: 1}
    prefix = 0
    count = 0
    for num in nums:
        prefix += num
        count += prefix_count.get(prefix - k, 0)
        prefix_count[prefix] = prefix_count.get(prefix, 0) + 1
    return count

Output

subarray_sum([1,1,1], 2) → 2
subarray_sum([1,2,3], 3) → 2 (subarrays [1,2] and [3])

Note Time O(n), Space O(n). Initialize map with {0: 1} to handle subarrays starting at index 0. This pattern is reusable: subarray sum divisible by k, subarray with equal 0s and 1s. One of the most asked medium-level problems.

Frequently asked questions

How does Interview Prep handle subarray sum?: Interview Prep covers this with 2 copy-ready snippets on this page. The "Prefix Sum" snippet in Interview Prep uses `Build a cumulative sum array so any range sum can be computed in O(1).`.
Which code does the Interview Prep example use?: The "Prefix Sum" snippet uses `Build a cumulative sum array so any range sum can be computed in O(1).`, from the Arrays & Strings section of the Interview Prep cheat sheet.
What other Interview Prep snippets are shown for "subarray sum"?: Besides "Prefix Sum", this page also shows "Subarray Sum Equals K".
Is there anything to watch out for?: Yes. For "Prefix Sum": Build: O(n) time, O(n) space. Query: O(1). Extremely useful when you need many range sum queries. Variant: prefix XOR for range XOR problems. For 2D grids, use 2D prefix sums with inclusion-exclusion.