Subarray sum equals K

Prefix sum with hash map.

prefix-sum dict arrays

Implement `subarray_sum(nums: list[int], k: int) -> int`.

Constraints

1 ≤ len(nums) ≤ 2×10^4; values can be negative.

Example

>>> subarray_sum([1,1,1], 2)
2

28 points ~25 min

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Hints

Track running prefix sum; count[prefix - k] gives subarrays ending here.

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