Subarray Sum Equals K — Python

# Time:  O(n)
# Space: O(n)

import collections


class Solution(object):
    def subarraySum(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: int
        """
        result = 0
        accumulated_sum = 0
        lookup = collections.defaultdict(int)
        lookup[0] += 1
        for num in nums:
            accumulated_sum += num
            result += lookup[accumulated_sum - k]
            lookup[accumulated_sum] += 1
        return result