LeetCode 560. Subarray Sum Equals K

Prefix Sum

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

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 * 104

• -1000 <= nums[i] <= 1000

• -107 <= k <= 107

Solution

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class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        unordered_map<int, int> prefix_sum;
        prefix_sum[0] = 1;
        
        int sum = 0;
        int ans = 0;
        for (int num : nums) {
            sum += num;
            int target = sum - k;
            ans += prefix_sum[target];
            prefix_sum[sum]++;
        }
        
        return ans;
    }
};