1588. Sum of All Odd Length Subarrays

Easy

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.

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

Follow up:

Could you solve this problem in O(n) time complexity?

解題

Runtime: 3 ms, faster than 59.29%

Memory Usage: 2.2 MB, less than 86.73%

func sumOddLengthSubarrays(arr []int) int {
    sum := 0
    for i:=1; i<=len(arr); i+=2 {
        for j:=0; j<=len(arr)-i; j++ {
            sum += sumup(arr[j:j+i])
        }
    }
    
    return sum
}

func sumup(arr []int) int {
    res := 0
    for _, num := range arr {
        res += num
    }
    
    return res
}