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Subarray Sum Equals K⚓︎

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Description⚓︎

Given an array of integers nums and an integer k, return the total number of subarrays whose sum equals to k.

A subarray is a contiguous non-empty sequence of elements within an array.

Example 1:

  • Input: nums = [1,1,1], k = 2
  • Output: 2

Example 2:

  • Input: nums = [1,2,3], k = 3
  • Output: 2

Constraints:

  • 1 <= nums.length <= 2 * 10^4
  • -1000 <= nums[i] <= 1000
  • -10^7 <= k <= 10^7

Solution⚓︎

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class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        unordered_map<int, int> sumToCount;
        sumToCount[0] = 1;
        int count = 0, prefixSum = 0;

        for (int x : nums) {
            prefixSum += x;
            if (sumToCount.find(prefixSum - k) != sumToCount.end()) {
                count += sumToCount[prefixSum - k];
            }
            sumToCount[prefixSum]++;
        }

        return count;
    }
};
  • Time complexity: \(O(n)\);
  • Space complexity: \(O(n)\).