Given an array of positive integers arr, calculate the sum of all possible odd-length subarrays.
A subarray is a contiguous subsequence of the array.
Return the sum of all odd-length subarrays of arr.

Example 1:

Input: arr = [1,4,2,5,3]
Output: 58
Explanation: The odd-length subarrays of arr and their sums are:
[1] = 1
[4] = 4
[2] = 2
[5] = 5
[3] = 3
[1,4,2] = 7
[4,2,5] = 11
[2,5,3] = 10
[1,4,2,5,3] = 15
If we add all these together we get 1 + 4 + 2 + 5 + 3 + 7 + 11 + 10 + 15 = 58
Example 2:

Input: arr = [1,2]
Output: 3
Explanation: There are only 2 subarrays of odd length, [1] and [2]. Their sum is 3.
Example 3:

Input: arr = [10,11,12]
Output: 66

解法：
详细解释：[C++] O(N) -Single Line Logic

class Solution {
public:
    int sumOddLengthSubarrays(vector<int>& arr) {
        int res = 0, n = arr.size();
        for (int i = 0; i < n; ++i) {
            res += ((i + 1) * (n - i) + 1) / 2 * arr[i];
        }
        return res;
    }
};