Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

Example 1:

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

Note:

1. The length of the array is in range [1, 20,000].
2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

Solution:

Prefix sum. Rather than keeping the sum, keep a map of count of a certain sum.

Now for any value, we add the number of cnt[sum-k] so the sum would be k.

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