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560. Subarray Sum Equals K

Array Hash Table Prefix Sum

Problem - Subarray Sum Equals K

Medium

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.

 

Example 1:

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

Example 2:

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

 

Constraints:

  • 1 <= nums.length <= 2 * 104
  • -1000 <= nums[i] <= 1000
  • -107 <= k <= 107

Solutions

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class Solution:
    def subarraySum(self, nums: List[int], k: int) -> int:
        count = Counter({0: 1})
        result = sum_val = 0

        for num in nums:
            sum_val += num  # extend the prefix sum
            result += count[sum_val - k] # how many valid subarrays end here?
            count[sum_val] += 1

        return result

Submission Stats:

  • Runtime: 51 ms (5.94%)
  • Memory: 21.9 MB (57.10%)