Given an array of positive integers arr, return the sum of all possible odd-length subarrays of arr.
A subarray is a contiguous subsequence of the array.
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Example 1:
\n\n\nInput: arr = [1,4,2,5,3]\nOutput: 58\nExplanation: The odd-length subarrays of arr and their sums are:\n[1] = 1\n[4] = 4\n[2] = 2\n[5] = 5\n[3] = 3\n[1,4,2] = 7\n[4,2,5] = 11\n[2,5,3] = 10\n[1,4,2,5,3] = 15\nIf we add all these together we get 1 + 4 + 2 + 5 + 3 + 7 + 11 + 10 + 15 = 58\n\n
Example 2:
\n\n\nInput: arr = [1,2]\nOutput: 3\nExplanation: There are only 2 subarrays of odd length, [1] and [2]. Their sum is 3.\n\n
Example 3:
\n\n\nInput: arr = [10,11,12]\nOutput: 66\n\n\n
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Constraints:
\n\n1 <= arr.length <= 1001 <= arr[i] <= 1000\n
Follow up:
\n\nCould you solve this problem in O(n) time complexity?
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