325. Maximum Size Subarray Sum Equals k

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Given an array nums and a target value k, find the maximum length of a subarray that sums to k. If there isn't one, return 0 instead.
Note:
The sum of the entire nums array is guaranteed to fit within the 32-bit signed integer range.
Example 1:
Input: nums = [1, -1, 5, -2, 3], k = 3
Output: 4 
Explanation: The subarray [1, -1, 5, -2] sums to 3 and is the longest.
Example 2:
Input: nums = [-2, -1, 2, 1], k = 1
Output: 2 
Explanation: The subarray [-1, 2] sums to 1 and is the longest.
Follow Up:
Can you do it in O(n) time?
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Intuition
Sub array sum => preprocess input to get range sum in const time => create prefix sum
Sum from i to j = prefix[j] - prefix[i - 1],  length of subarray = j - 1

Problem reduces to Max (j - i) such that prefix[j] - prefix[i] = k

Instead of brute force, we can lookup complement prefix[index] - k from hash map
Hash Map < Prefix Sum, Index>
Seed with sum = 0, index = -1

Also save the current prefix sum and index as we traverse L to R
Only save first index, do not overwrite since we need longest subarray
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Time - O(N)
Space - O(N)
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Related problems
560-subarray-sum-equals-k
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