LeetCode 3701: Compute Alternating Sum

Meta Data

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Difficulty: easy First Attempt: 2025-10-05

• Submissions: Accepted (first try)

Approach

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Sum the even positions and the odd positions separately; their difference is the alternating sum (Array Template 5).

My Solution

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class Solution:
    def alternatingSum(self, nums: List[int]) -> int:
        return sum(nums[::2]) - sum(nums[1::2])

I originally added or subtracted in a loop based on i % 2; same result.

Complexity

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Time O(n), space O(n) (slices).

Mistakes to Avoid

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• Indices start at 0, so the first element is added.

Interview Explanation

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Sum the even positions, sum the odd positions, and subtract. Time O(n).