Skip to main content

24. Subarray Sum Equals K

mediumAsked at Databricks

Count contiguous subarrays whose values sum to k — the prefix-sum technique here is the same one Databricks uses to compute rolling aggregations over unbounded streaming windows in Structured Streaming.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an array of integers nums and an integer k, return the total number of subarrays whose sum equals k.

Constraints

  • 1 <= nums.length <= 2 * 10^4
  • -1000 <= nums[i] <= 1000
  • -10^7 <= k <= 10^7

Examples

Example 1

Input
nums = [1,1,1], k = 2
Output
2

Explanation: Subarrays [1,1] at indices 0-1 and 1-2 both sum to 2.

Example 2

Input
nums = [1,2,3], k = 3
Output
2

Explanation: [1,2] and [3] each sum to 3.

Approaches

1. Brute force — all subarrays

For each pair (i, j), compute the subarray sum and compare to k. O(n^2) time — impractical for large streaming windows.

Time
O(n^2)
Space
O(1)
function subarraySum(nums, k) {
  let count = 0;
  for (let i = 0; i < nums.length; i++) {
    let sum = 0;
    for (let j = i; j < nums.length; j++) {
      sum += nums[j];
      if (sum === k) count++;
    }
  }
  return count;
}

Tradeoff:

2. Prefix sum with hash map

Maintain a running prefix sum and a map from sum → count of times seen. For each position, the number of valid subarrays ending here equals the count of earlier prefix sums equal to (currentSum - k).

Time
O(n)
Space
O(n)
function subarraySum(nums, k) {
  const prefixCount = new Map([[0, 1]]);
  let sum = 0, count = 0;
  for (const num of nums) {
    sum += num;
    count += prefixCount.get(sum - k) || 0;
    prefixCount.set(sum, (prefixCount.get(sum) || 0) + 1);
  }
  return count;
}

Tradeoff:

Databricks-specific tips

This problem is a frequent Databricks phone screen fixture because it tests prefix-sum intuition — a building block of their Photon query engine's aggregate operators. Emphasize that the map must be seeded with {0: 1} before processing: without it, subarrays starting at index 0 are missed. Interviewers will ask a follow-up about negative numbers — the prefix-sum approach handles them correctly, unlike a sliding-window approach, so address that proactively.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Subarray Sum Equals K and other Databricks interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →