Sum of All Odd Length Subarrays

    vector<int> arr = {1,4,2,5,3};
    int size = arr.size();
    int sum = 0;
    vector<int> prefix(size+1);
    for(int i =1;i<=size;i++)
    {
        prefix[i] = prefix[i-1] + arr[i-1];
    }
    for(int i =1;i<=size;i++)
    {
        for(int j=i;j<=size;j++)
        {
            if(((j-i) + 1) %2 == 1)
            {
                sum += (prefix[j] - prefix[i-1]);
            }
        }
    }
    return sum;