LeetCode-in-All

560. Subarray Sum Equals K

Medium

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.

Example 1:

Input: nums = [1,1,1], k = 2

Output: 2

Example 2:

Input: nums = [1,2,3], k = 3

Output: 2

Constraints:

• 1 <= nums.length <= 2 * 104
• -1000 <= nums[i] <= 1000
• -107 <= k <= 107

Solution

-spec subarray_sum(Nums :: [integer()], K :: integer()) -> integer().
subarray_sum(Nums, K) ->
    subarray_sum(Nums, K, 0, 0, #{0 => 1}).

-spec subarray_sum([integer()], integer(), integer(), integer(), #{integer() => integer()}) -> integer().
subarray_sum([], _K, _TempSum, Ret, _SumCount) -> 
    Ret;
subarray_sum([Num | Rest], K, TempSum, Ret, SumCount) ->
    NewTempSum = TempSum + Num,
    Count = maps:get(NewTempSum - K, SumCount, 0),
    NewRet = Ret + Count,
    UpdatedCount = maps:get(NewTempSum, SumCount, 0) + 1,
    NewSumCount = maps:put(NewTempSum, UpdatedCount, SumCount),
    subarray_sum(Rest, K, NewTempSum, NewRet, NewSumCount).

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