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Subarray Sum Equals K
Given an array of integers nums and an integer k, return the total number of contiguous subarrays whose sum equals k.
The canonical O(n) solution walks a running prefix sum and asks, at each index, "how many earlier prefixes equal prefix - k?" — answered in O(1) by a hash map keyed on prefix-sum frequencies (seeded with {0: 1} to count subarrays that start at index 0).
Examples
Constraints
- 1 <= nums.length <= 2 * 10^4
- -1000 <= nums[i] <= 1000
- -10^7 <= k <= 10^7
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Your approach
- Pattern:Prefix Sum
- Time:O(N)
- Space:O(1)
Complete approach — pattern and complexity both named.
Coach
What you got right
You correctly matched the sorted input requirement and implemented a linear-time scan using two pointers.
Where it diverged
No divergence detected. The code correctly aligns with the stated approach.
Next attempt: focus on
Try solving related sliding window problems to build familiarity with two-pointer variants.
Signals
No signals fired — clean run, you stayed in flow.