Skip to main content

20. Subarray Sum Equals K

mediumAsked at Byju's

Count the number of contiguous subarrays whose sum equals k.

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 to k. A subarray is a contiguous non-empty sequence of elements within an array. The array may include negative values.

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

Example 2

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

Approaches

1. Nested sum

Enumerate every (i,j) pair and accumulate the sum.

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

Tradeoff:

2. Prefix sum with hash count

Track running prefix sums in a Map of prefixSum -> occurrences. For each index, look up (prefix - k) to count valid subarrays ending here.

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

Tradeoff:

Byju's-specific tips

Byju's K-12 analytics computes engagement windows on prefix-sum aggregates, so frame the hash-prefix trick as 'rolling-watch-time matching'.

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 Byju's interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →