560. Subarray Sum Equals K

[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.

 

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 * 10^4
• -1000 <= nums[i] <= 1000
• -107 <= k <= 107

[Solution]

class Solution:
    def subarraySum(self, nums: List[int], k: int) -> int:
        ret = 0
        prefix_sum = 0
        m = {0: 1}
        for num in nums:
            prefix_sum = prefix_sum + num
            if prefix_sum - k in m:
                ret += m[prefix_sum - k]
            if prefix_sum not in m:
                m[prefix_sum] = 1
            else:
                m[prefix_sum] += 1
        return ret