2090. K Radius Subarray Averages

MediumView on LeetCode

Array Sliding Window

Time: O(n)

Space: O(n)

Intuition

K-radius average: for each index i, average of nums[i-k..i+k]. Use prefix sums for O(n) computation.

Algorithm

1prefix[i] = sum of nums[0..i-1].
2For each i from k to n-1-k: avg = (prefix[i+k+1] - prefix[i-k]) / (2k+1).
3Positions < k or >= n-k get -1.

Common Pitfalls

•Use integer division. Return -1 for positions without full radius window.

2090.cs

// Approach: Prefix sums; fill averages for indices with full window of size 2k+1.
// Time: O(n) Space: O(n)

public class Solution
{
    public int[] GetAverages(int[] nums, int k)
    {
        int n = nums.Length;
        int size = 2 * k + 1;
        int[] ans = new int[n];
        Array.Fill(ans, -1);
        if (size > n)
            return ans;

        long sum = 0;

        for (int i = 0; i < size; ++i)
            sum += nums[i];

        for (int i = k; i + k < n; ++i)
        {
            ans[i] = (int)(sum / size);
            if (i + k + 1 < n)
                sum += nums[i + k + 1] - nums[i - k];
        }

        return ans;
    }
}

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