Скачать или смотреть LeetCode 560 | Subarray Sum Equals K

Problem Statement:

Given an integer array and a number k, we have to find the number of subarrays whose sum equals exactly k.

Approach 1: Brute Force – O(n²)

✅ We use two nested loops to generate all subarrays

✅ For each subarray, we calculate the sum

✅ If sum == k → increase count

Approach 2: Optimized using Prefix Sum + HashMap – O(n)

✅ Maintain a running sum (prefixSum)

✅ Use a HashMap to store frequency of prefixSums

✅ If (prefixSum - k) exists in map → we found a valid subarray

✅ Add its frequency to the answer


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