1588. Sum of All Odd Length Subarrays

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

# @lc code=start
using LeetCode

function sum_odd_length_subarrays(arr::Vector{Int})::Int
    res = 0
    len = length(arr)
    for i in 1:len
        l, r = i - 1, len - i
        lodd, rodd = (l + 1) ÷ 2, (r + 1) ÷ 2
        leven, reven = l ÷ 2 + 1, r ÷ 2 + 1
        res += arr[i] * (lodd * rodd + leven * reven)
    end
    res
end
# @lc code=end

sum_odd_length_subarrays (generic function with 1 method)

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