input: Write a python function to find the number of continuous subarrays whose sum equals to k.
  assert subarraySum([1, 1, 1], 2) == 2
  assert subarraySum([1, 2, 3], 3) == 2
  assert subarraySum([1, -1, 1, 1, 1], 2) == 4
output: def subarraySum(nums, k):
      count = 0
      curr_sum = 0
      prefix_sums = {0: 1}
      
      for num in nums:
          curr_sum += num
          if curr_sum - k in prefix_sums:
              count += prefix_sums[curr_sum - k]
          if curr_sum in prefix_sums:
              prefix_sums[curr_sum] += 1
          else:
              prefix_sums[curr_sum] = 1
      
      return count