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
Constraints:
1 <= arr.length <= 100
1 <= arr[i] <= 1000
Solution:
class Solution {
public int sumOddLengthSubarrays(int[] arr) {
// [1,4,2,5,3]
// 3 4 5 4 3
// [10 11 12]
// 2 2 2
int n = arr.length;
int sum = 0;
for (int i = 0; i < n; i ++) {
int endI = i + 1;
int startI = n - i;
int totalIncludeI = endI * startI;
int odd = totalIncludeI / 2;
if (totalIncludeI % 2 == 1) odd ++;
sum += arr[i] * odd;
}
return sum;
}
}