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560. Subarray Sum Equals K

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	`1`	+/// This function has a time complexity of `O(n)`,
	`2`	+/// where `n` is the length of `nums`. This is because
	`3`	`+/// the function iterates through nums once and`
	`4`	`+/// performs constant time operations on each element.`
	`5`	`+/// The function also uses a map to store the prefix sum,`
	`6`	+/// which has a space complexity of `O(n)` in the worst case,
	`7`	+/// where `n` is the length of `nums`.
	`8`	`+`
	`9`	`+class Solution {`
	`10`	`+ int subarraySum(List<int> nums, int k) {`
	`11`	`+ int count = 0;`
	`12`	`+ int sum = 0;`
	`13`	`+ Map<int, int> prefixSum = {0: 1};`
	`14`	`+`
	`15`	`+ for (int i = 0; i < nums.length; i++) {`
	`16`	`+ sum += nums[i];`
	`17`	`+ if (prefixSum.containsKey(sum - k)) {`
	`18`	`+ count += prefixSum[sum - k]!;`
	`19`	`+ }`
	`20`	`+ prefixSum[sum] = (prefixSum[sum] ?? 0) + 1;`
	`21`	`+ }`
	`22`	`+`
	`23`	`+ return count;`
	`24`	`+ }`
	`25`	`+}`
	`26`	`+`
	`27`	`+void main(List<String> args) {`
	`28`	`+ print(Solution().subarraySum([-1, -1, 1], 0));`
	`29`	`+ print(Solution().subarraySum([1, 2, 5, 3, 4], 6));`
	`30`	`+ print(Solution().subarraySum([1, 1, 1], 2));`
	`31`	`+ print(Solution().subarraySum([1], 1));`
	`32`	`+ print(Solution().subarraySum([-1, 2, -1, -2, -3], -3));`
	`33`	`+}`

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	`1`	`+560. Subarray Sum Equals K`
	`2`	`+https://leetcode.com/problems/subarray-sum-equals-k/`
	`3`	`+`
	`4`	`+Given an array of integers nums and an integer k,`
	`5`	`+return the total number of subarrays whose sum equals to k.`
	`6`	`+A subarray is a contiguous non-empty sequence of elements within an array.`
	`7`	`+`
	`8`	`+`
	`9`	`+Example 1:`
	`10`	`+`
	`11`	`+Input: nums = [1,1,1], k = 2`
	`12`	`+Output: 2`
	`13`	`+`
	`14`	`+Example 2:`
	`15`	`+`
	`16`	`+Input: nums = [1,2,3], k = 3`
	`17`	`+Output: 2`
	`18`	`+`
	`19`	`+`
	`20`	`+Constraints:`
	`21`	`+`
	`22`	`+1 <= nums.length <= 2 * 104`
	`23`	`+-1000 <= nums[i] <= 1000`
	`24`	`+-107 <= k <= 107`

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