560. Subarray Sum Equals K

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https://leetcode.com/problems/subarray-sum-equals-k/

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.
Example 1:
Input:nums = [1,1,1], k = 2
Output: 2
Note:
  1. The length of the array is in range [1, 20,000].
  2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].
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Intuition

If prefixSum[j] - prefixSum[i] == 0 => elements i..j did not change prefix sum => sum(arr[i...j]) = 0

Extending this
If prefixSum[j] - prefixSum[i] == k => sum(arr[i...j]) = k

Track prefixSum, and # of times it occurs in HashMap

Iterate through the array
Compute prefix sum

# of times (prefixSum - k) has occurred up-to this point = # of times subarrays sum to k before current => add to answer

Update hashmap with freq of current prefix Sum
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Time - O(n)
Space - O(n)