k_radius_subarray_averages.dart

/*


-* 2090. K Radius SubArray Averages *-



You are given a 0-indexed array nums of n integers, and an integer k.

The k-radius average for a subarray of nums centered at some index i with the radius k is the average of all elements in nums between the indices i - k and i + k (inclusive). If there are less than k elements before or after the index i, then the k-radius average is -1.

Build and return an array avgs of length n where avgs[i] is the k-radius average for the subarray centered at index i.

The average of x elements is the sum of the x elements divided by x, using integer division. The integer division truncates toward zero, which means losing its fractional part.

For example, the average of four elements 2, 3, 1, and 5 is (2 + 3 + 1 + 5) / 4 = 11 / 4 = 2.75, which truncates to 2.
 

Example 1:


Input: nums = [7,4,3,9,1,8,5,2,6], k = 3
Output: [-1,-1,-1,5,4,4,-1,-1,-1]
Explanation:
- avg[0], avg[1], and avg[2] are -1 because there are less than k elements before each index.
- The sum of the subarray centered at index 3 with radius 3 is: 7 + 4 + 3 + 9 + 1 + 8 + 5 = 37.
  Using integer division, avg[3] = 37 / 7 = 5.
- For the subarray centered at index 4, avg[4] = (4 + 3 + 9 + 1 + 8 + 5 + 2) / 7 = 4.
- For the subarray centered at index 5, avg[5] = (3 + 9 + 1 + 8 + 5 + 2 + 6) / 7 = 4.
- avg[6], avg[7], and avg[8] are -1 because there are less than k elements after each index.
Example 2:

Input: nums = [100000], k = 0
Output: [100000]
Explanation:
- The sum of the subarray centered at index 0 with radius 0 is: 100000.
  avg[0] = 100000 / 1 = 100000.
Example 3:

Input: nums = [8], k = 100000
Output: [-1]
Explanation: 
- avg[0] is -1 because there are less than k elements before and after index 0.
 

Constraints:

n == nums.length
1 <= n <= 105
0 <= nums[i], k <= 105

*/

class A {
  List<int> getAverages(List<int> nums, int k) {
    final int n = nums.length;
    final int windowSize = 2 * k + 1;
    final List<int> ans = List.filled(n, -1);

    if (n < windowSize) return ans;

    final List<int> prefixSum = List.filled(n + 1, 0);
    for (int i = 0; i < n; ++i) prefixSum[i + 1] = prefixSum[i] + nums[i];

    for (int i = k; i + k < n; ++i)
      ans[i] = ((prefixSum[i + k + 1] - prefixSum[i - k]) ~/ windowSize);

    return ans;
  }
}

class B {
  List<int> getAverages(List<int> nums, int k) {
    final int n = nums.length;
    final int windowSize = 2 * k + 1;

    int windowSum = 0;
    final List<int> result = List.filled(n, -1);

    if (n < windowSize) return result;

    for (int i = 0; i < n; ++i) {
      windowSum += nums[i];

      if (i - windowSize >= 0) windowSum -= nums[i - windowSize];

      if (i >= windowSize - 1) result[i - k] = windowSum ~/ windowSize;
    }

    return result;
  }
}