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+# 560. 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.
+"""
+
+
+class Solution(object):
+ def subarraySum(self, nums, k):
+ ans = 0
+ prefix_sum = 0
+ d = {0: 1}
+ for n in nums:
+ prefix_sum += n
+ if prefix_sum - k in d:
+ ans += d[prefix_sum - k]
+ if prefix_sum not in d:
+ d[prefix_sum] = 1
+ else:
+ d[prefix_sum] += 1
+ return ans
+
+
+if __name__ == "__main__":
+ obj = Solution()
+ print(obj.subarraySum([1, 1, 1], 2)) # expect: 2
+ print(obj.subarraySum([1, 2, 3], 3)) # expect: 2