class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        int n = nums.size();
        vector<int> p(n+1, 0);
        for(int i = 1; i <= n; i++){
            p[i] = p[i - 1] + nums[i - 1];
        }
        int ret = 0;
        unordered_map<int, int> data;
        for(int l = n; l; l--){
            data[p[l]]++;
            int target = k + p[l-1];
            if(data.count(target)){
                ret += data[target];
            }
        }
        return ret;
    }
};