Subarray Sum Equals K

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

SOLUTION:

class Solution:
    def subarraySum(self, nums: List[int], k: int) -> int:
        n = len(nums)
        curr = 0
        prevsum = {}
        ctr = 0
        for num in nums:
            curr += num
            if curr == k:
                ctr += 1
            if (curr - k) in prevsum:
                ctr += prevsum[curr - k]
            prevsum[curr] = prevsum.get(curr, 0) + 1
        return ctr