0560 - Subarray Sum Equals K (Medium)

Problem Link

https://leetcode.com/problems/subarray-sum-equals-k/

Problem Statement

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 * 10^4
• -1000 <= nums[i] <= 1000
• -10^7 <= k <= 10^7

Approach 1: Hash Map

We use hash map to store the cumulative sum $sum[i]$ up to index $i$. If $sum[i] - sum[j] = k$, then it means the sum between indices $i$ and $j$ is $k$. Therefore, we store the cumulative sum, search for $sum - k$ in the hash map and add the occurrences if it is found to the answer.

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        unordered_map<int, int> m;
        m[0]++;
        int sum = 0, ans = 0;
        for (int i = 0; i < nums.size(); i++) {
            sum += nums[i];
            ans += m[sum - k];
            m[sum]++;
        }
        return ans;
    }
};