subArraySumEqualsK

//for ith element if currsum == k there may be possible that currsum-num=k=>currsum-k=num i.e. there exists num in prefix sum array which can result to subarray sum k.
class Solution {
public:
    int subarraySum(vector<int>& a, int k) {
        unordered_map<int,int>mp;
        int ans = 0,sum=0;
        for(int i=0;i<a.size();i++)
        {
           sum+=a[i];
           if(sum==k)
           {
               ans++;
           }
           //check for every case
           if(mp.find(sum-k)!=mp.end())
            {
               ans+=mp[sum-k];
            }
           mp[sum]++;
        }
       return ans;
    }
};