subarray-sum-equals-k.py

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

# Given an array of integers and an integer k,
# you need to find the total number of continuous subarrays whose sum equals to k.
#
# Example 1:
# Input:nums = [1,1,1], k = 2
# Output: 2
#
# Note:
# The length of the array is in range [1, 20,000].
# The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

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